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Compiled Tips ‘N Tricks Guide

Information contained in this publication regarding device

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intended manner and under normal conditions.

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8-pin Flash PIC

Microcontroller Tips ‘n Tricks

Tips ‘n Tricks

TABLE OF CONTENTS

8-pin Flash PIC® Microcontrollers

Tips ‘n Tricks

TIPS ‘N TRICKS WITH HARDWARE

TIP #1: Dual Speed RC Oscillator .......................... 1-2

TIP #2: Input/Output Multiplexing............................ 1-2

TIP #3: Read Three States From One Pin .............. 1-3

TIP #4: Reading DIP Switches ................................ 1-3

TIP #5: Scanning Many Keys With One Input......... 1-4

TIP #6: Scanning Many Keys and Wake-up

From Sleep ................................................. 1-4

TIP #7: 8x8 Keyboard with 1 Input .......................... 1-5

TIP #8: One Pin Power/Data................................... 1-5

TIP #9: Decode Keys and ID Settings .................... 1-6

TIP #10: Generating High Voltages .......................... 1-6

TIP #11: V Self Starting Circuit ............................. 1-7dd

TIP #12: Using PIC

MCU A/D For Smart

Current Limiter............................................ 1-7

TIP #13: Reading A Sensor With Higher Accuracy ... 1-8

TIP #13.1: Reading A Sensor With Higher

Accuracy – RC Timing Method ................... 1-8

TIP #13.2: Reading A Sensor With Higher

Accuracy – Charge Balancing Method .......1-10

TIP #13.3: Reading A Sensor With Higher

Accuracy – A/D Method ..............................1-11

TIP #14: Delta Sigma Converter ...............................1-11

TIPS ‘N TRICKS WITH SOFTWARE

TIP #15: Delay Techniques .......................................1-12

TIP #16: Optimizing Destinations..............................1-13

TIP #17: Conditional Bit Set/Clear ............................1-13

TIP #18: Swap File Register with W .........................1-14

TIP #19: Bit Shifting Using Carry Bit .........................1-14

PIC® Microcontroller Low Power

Tips ‘n Tricks

GENERAL LOW POWER TIPS ‘N TRICKS

TIP #1 Switching Off External Circuits/

Duty Cycle .................................................. 2-2

TIP #2 Power Budgeting ........................................ 2-3

TIP #3 Conguring Port Pins ................................. 2-4

TIP #4 Use High-Value Pull-Up Resistors .............. 2-4

TIP #5 Reduce Operating Voltage ......................... 2-4

TIP #6 Use an External Source for

CPU Core Voltage ...................................... 2-5

TIP #7 Battery Backup for PIC MCUs ................... 2-6

DYNAMIC OPERATION TIPS ‘N TRICKS

TIP #8 Enhanced PIC16 Mid-Range Core ............. 2-6

TIP #9 Two-Speed Start-Up ................................... 2-7

TIP #10 Clock Switching .......................................... 2-7

TIP #11 Use Internal RC Oscillators ........................ 2-7

TIP #12 Internal Oscillator Calibration ..................... 2-8

TIP #13 Idle and Doze Modes ................................. 2-8

TIP #14 Use and Idle Mode .............................. 2-9NOP

TIP #15 Peripheral Module Disable

(PMD) Bits .................................................. 2-9

STATIC POWER REDUCTION TIPS ‘N TRICKS

TIP #16 Deep Sleep Mode.......................................2-10

TIP #17 Extended WDT and Deep

Sleep WDT .................................................2-10

TIP #18 Low Power Timer1 Oscillator and RTCC ....2-10

TIP #19 Low Power Timer1 Oscillator Layout ..........2-11

TIP #20 Use LVD to Detect Low Battery ..................2-11

TIP #21 Use Peripheral FIFO and DMA...................2-11

TIP #22 Ultra Low-Power

Wake-Up Peripheral ...................................2-12

PIC® Microcontroller CCP and ECCP

Tips ‘n Tricks

CAPTURE TIPS ‘N TRICKS

TIP #1: Measuring the Period of a Square Wave ... 3-3

TIP #2: Measuring the Period of a

Square Wave with Averaging ..................... 3-3

TIP #3: Measuring Pulse Width .............................. 3-4

TIP #4: Measuring Duty Cycle ................................ 3-4

TIP #5: Measuring RPM Using an Encoder ............ 3-5

TIP #6: Measuring the Period of an Analog Signal 3-6 .

COMPARE TIPS ‘N TRICKS

TIP #7: Periodic Interrupts ...................................... 3-8

TIP #8: Modulation Formats.................................... 3-9

TIP #9: Generating the Time Tick for a RTOS ........3-10

TIP #10: 16-Bit Resolution PWM ..............................3-10

TIP #11: Sequential ADC Reader .............................3-11

TIP #12: Repetitive Phase Shifted Sampling ............3-12

PWM TIPS ‘N TRICKS

TIP #13: Deciding on PWM Frequency.....................3-14

TIP #14: Unidirectional Brushed DC

Motor Control Using CCP ...........................3-14

TIP #15: Bidirectional Brushed DC

Motor Control Using ECCP ........................3-15

TIP #16: Generating an Analog Output .....................3-16

TIP #17: Boost Power Supply ...................................3-17

TIP #18: Varying LED Intensity .................................3-18

TIP #19: Generating X-10 Carrier Frequency ...........3-18

COMBINATION CAPTURE AND COMPARE TIPS

TIP #20: RS-232 Auto-baud ......................................3-19

TIP #21: Dual-Slope Analog-to-Digital Converter .....3-21

Tips ‘n Tricks Table of Contents

PIC® Microcontroller Comparator

Tips ‘n Tricks

TIP #1: Low Battery Detection ................................ 4-2

TIP #2: Faster Code for Detecting Change............. 4-3

TIP #3: Hysteresis................................................... 4-4

TIP #4: Pulse Width Measurement ......................... 4-5

TIP #5: Window Comparison .................................. 4-6

TIP #6: Data Slicer .................................................. 4-7

TIP #7: One-Shot .................................................... 4-8

TIP #8: Multi-Vibrator (Square Wave Output) ......... 4-9

TIP #9: Multi-Vibrator (Ramp Wave Output) ...........4-10

TIP #10: Capacitive Voltage Doubler ........................4-11

TIP #11: PWM Generator .........................................4-12

TIP #12: Making an Op Amp Out of a Comparator ...4-13

TIP #13: PWM High-Current Driver ..........................4-14

TIP #14: Delta-Sigma ADC .......................................4-15

TIP #15: Level Shifter ...............................................4-16

TIP #16: Logic: Inverter.............................................4-16

TIP #17: Logic: AND/NAND Gate .............................4-17

TIP #18: Logic: OR/NOR Gate..................................4-18

TIP #19: Logic: XOR/XNOR Gate .............................4-19

TIP #20: Logic: Set/Reset Flip Flop ..........................4-20

PIC® Microcontroller DC Motor Control

Tips ‘n Tricks

TIP #1: Brushed DC Motor Drive Circuits ............... 5-2

TIP #2: Brushless DC Motor Drive Circuits ............. 5-3

TIP #3: Stepper Motor Drive Circuits ...................... 5-4

TIP #4: Drive Software ............................................ 5-6

TIP #5: Writing a PWM Value to the CCP

Registers with a Mid-Range PIC

MCU ..... 5-7

TIP #6: Current Sensing ......................................... 5-8

TIP #7: Position/Speed Sensing ............................. 5-9

Application Note References .........................................5-11

Motor Control Development Tools .................................5-11

LCD PIC® Microcontroller Tips ‘n Tricks

TIP #1: Typical Ordering Considerations and

Procedures for Custom Liquid Displays ..... 6-2

TIP #2: LCD PIC

MCU Segment/Pixel Table ......... 6-2

TIP #3: Resistor Ladder for Low Current ................ 6-3

TIP #4: Contrast Control with a Buck Regulator ..... 6-5

TIP #5: Contrast Control Using a Boost

Regulator .................................................... 6-5

TIP #6: Software Controlled Contrast with

PWM for LCD Contrast Control .................. 6-6

TIP #7: Driving Common Backlights ....................... 6-7

TIP #8: In-Circuit Debug (ICD) ................................ 6-8

TIP #9: LCD in Sleep Mode .................................... 6-8

TIP #10: How to Update LCD Data

Through Firmware ...................................... 6-9

TIP #11: Blinking LCD............................................... 6-9

TIP #12: 4 x 4 Keypad Interface that Conserves

Pins for LCD Segment Drivers ...................6-10

Application Note References .........................................6-11

Intelligent Power Supply Design

Tips ‘n Tricks

TIP #1: Soft-Start Using a PIC10F200 .................... 7-2

TIP #2: A Start-Up Sequencer ................................ 7-3

TIP #3: A Tracking and Proportional

Soft-Start of Two Power Supplies ............... 7-4

TIP #4: Creating a Dithered PWM Clock ................ 7-5

TIP #5: Using a PIC

Microcontroller as a Clock

Source for a SMPS PWM Generator.......... 7-6

TIP #6: Current Limiting Using the MCP1630 ......... 7-7

TIP #7: Using a PIC

Microcontroller for

Power Factor Correction ............................ 7-8

TIP #8: Transformerless Power Supplies ............... 7-9

TIP #9: An IR Remote Control Actuated AC

Switch for Linear Power Supply Designs ...7-10

TIP #10: Driving High Side FETs ..............................7-11

TIP #11: Generating a Reference Voltage with a

PWM Output ...............................................7-12

TIP #12: Using Auto-Shutdown CCP ........................7-13

TIP #13: Generating a Two-Phase Control Signal ....7-14

TIP #14: Brushless DC Fan Speed Control ..............7-15

TIP #15: High Current Delta-Sigma Based Current

Measurement Using a Slotted Ferrite

and Hall Effect Device ................................7-16

TIP #16: Implementing a PID Feedback Control

in a PIC12F683-Based SMPS Design........7-17

TIP #17: An Error Detection and Restart Controller..7-18

TIP #18: Data-Indexed Software State Machine.......7-19

TIP #19: Execution Indexed Software

State Machine ............................................7-20

TIP #20: Compensating Sensors Digitally ................7-21

TIP #21: Using Output Voltage Monitoring to

Create a Self-Calibration Function .............7-22

3V Tips ‘n Tricks

TIP #1: Powering 3.3V Systems From 5V

Using an LDO Regulator ............................ 8-3

TIP #2: Low-Cost Alternative Power System

Using a Zener Diode .................................. 8-4

TIP #3: Lower Cost Alternative Power System

Using 3 Rectier Diodes ............................. 8-4

TIP #4: Powering 3.3V Systems From 5V

Using Switching Regulators ....................... 8-5

TIP #5: 3.3V → 5V Direct Connect ......................... 8-6

TIP #6: 3.3V → 5V Using a MOSFET Translator .... 8-6

TIP #7: 3.3V → 5V Using A Diode Offset ................ 8-7

TIP #8: 3.3V → 5V Using A Voltage Comparator .... 8-8

TIP #9: 5V → 3.3V Direct Connect ......................... 8-9

TIP #10: 5V → 3.3V With Diode Clamp .................... 8-9

TIP #11: 5V → 3.3V Active Clamp ............................8-10

TIP #12: 5V → 3.3V Resistor Divider........................8-10

TIP #13: 3.3V → 5V Level Translators......................8-12

TIP #14: 3.3V → 5V Analog Gain Block....................8-13

TIP #15: 3.3V → 5V Analog Offset Block ..................8-13

TIP #16: 5V → 3.3V Active Analog Attenuator ..........8-14

TIP #17: 5V → 3V Analog Limiter .............................8-15

TIP #18: Driving Bipolar Transistors .........................8-16

TIP #19: Driving N-Channel MOSFET Transistors ...8-18

8-pin Flash PIC

Microcontroller Tips ‘n Tricks

CHAPTER 1

8-Pin Flash PIC® Microcontrollers

Tips ‘n Tricks

Table Of Contents

TIPS ‘N TRICKS WITH HARDWARE

TIP #1: Dual Speed RC Oscillator ................ 1-2

TIP #2: Input/Output Multiplexing .................. 1-2

TIP #3: Read Three States From One Pin .... 1-3

TIP #4: Reading DIP Switches ...................... 1-3

TIP #5: Scanning Many Keys With

One Input .......................................... 1-4

TIP #6: Scanning Many Keys and Wake-up

From Sleep ....................................... 1-4

TIP #7: 8x8 Keyboard with 1 Input ................ 1-5

TIP #8: One Pin Power/Data ......................... 1-5

TIP #9: Decode Keys and ID Settings .......... 1-6

TIP #10: Generating High Voltages ................ 1-6

TIP #11: V Self Starting Circuit.................... 1-7dd

TIP #12: Using PIC® MCU A/D For Smart

Current Limiter .................................. 1-7

TIP #13: Reading A Sensor With Higher

Accuracy ........................................... 1-8

TIP #13.1: Reading A Sensor With Higher

Accuracy – RC Timing Method ......... 1-8

TIP #13.2: Reading A Sensor With Higher

Accuracy – Charge Balancing

Method ............................................. 1-10

TIP #13.3: Reading A Sensor With Higher

Accuracy – A/D Method .................... 1-11

TIP #14: Delta Sigma Converter ..................... 1-11

TIPS ‘N TRICKS WITH SOFTWARE

TIP #15: Delay Techniques ............................. 1-12

TIP #16: Optimizing Destinations .................... 1-13

TIP #17: Conditional Bit Set/Clear .................. 1-13

TIP #18: Swap File Register with W ............... 1-14

TIP #19: Bit Shifting Using Carry Bit ............... 1-14

TIPS ‘N TRICKS INTRODUCTION

Microchip continues to provide innovative

products that are smaller, faster, easier to

use and more reliable. The 8-pin Flash PIC®

microcontrollers (MCU) are used in an wide

range of everyday products, from toothbrushes,

hair dryers and rice cookers to industrial,

automotive and medical products.

The PIC12F629/675 MCUs merge all the

advantages of the PIC MCU architecture and

the exibility of Flash program memory into

an 8-pin package. They provide the features

and intelligence not previously available due

to cost and board space limitations. Features

include a 14-bit instruction set, small footprint

package, a wide operating voltage of 2.0 to

5.5 volts, an internal programmable 4 MHz

oscillator, on-board EEPROM data memory,

on-chip voltage reference and up to 4 channels

of 10-bit A/D. The exibility of Flash and an

excellent development tool suite, including

a low-cost In-Circuit Debugger, In-Circuit

Serial Programming™ and MPLAB® ICE 2000

emulation, make these devices ideal for just

about any embedded control application.

TIPS ‘N TRICKS WITH HARDWARE

The following series of Tips ’n Tricks can be

applied to a variety of applications to help make

the most of the 8-pin dynamics.

8-pin Flash PIC

Microcontroller Tips ‘n Tricks

TIP #1 Dual Speed RC Oscillator

Figure 1-1

PIC12F6XX

OSC1

GP0

+5V

1. After reset I/O pin is High-Z

2. Output ‘1’ on I/O pin

3. R1, R2 and C determine OSC frequency

4. Also works with additional capacitors

Frequency of PIC MCU in external RC oscillator

mode depends on resistance and capacitance

on OSC1 pin. Resistance is changed by the

output voltage on GP0. GP0 output ‘1’ puts R2

in parallel with R1 reduces OSC1 resistance

and increases OSC1 frequency. GP0 as

an input increases the OSC1 resistance

by minimizing current ow through R2, and

decreases frequency and power consumption.

Summary:

GP0 = Input: Slow speed for low current

GP0 = Output high: High speed for fast

processing

TIP #2 Input/Output Multiplexing

Individual diodes and some combination of

diodes can be enabled by driving I/Os high and

low or switching to inputs (Z). The number of

diodes (D) that can be controlled depends on

the number of I/Os (GP) used.

The equation is: D = GP x (GP - 1).

Example 2-1: Six LEDs on Three I/O Pins

GPx LEDs

0 1 2

0 0 0

0 1 Z

1 0 Z

Z 0 1

Z 1 0

0 Z 1

1 Z 0

0 0 1

0 1 0

0 1 1

1 0 0

1 0 1

1 1 0

1 1 1

1 2 3 4 5 6

0 0 0 0 0 0

1 0 0 0 0 0

0 1 0 0 0 0

0 0 1 0 0 0

0 0 0 1 0 0

0 0 0 0 1 0

0 0 0 0 0 1

0 0 1 0 1 0

1 0 0 1 0 0

1 0 0 0 1 0

0 1 0 0 0 1

0 1 1 0 0 0

0 0 0 1 0 1

0 0 0 0 0 0

Figure 2-1

PIC12F6XX

1 2 5

GP0

GP1

GP2

8-pin Flash PIC

Microcontroller Tips ‘n Tricks

TIP #3 Read Three States

From One Pin

To check state Z:

• Drive output pin high

• Set to Input

• Read 1

• Drive output pin low

• Set to Input

• Read 0

To check state 0:

• Read 0 on pin

To check state 1:

• Read 1 on pin

State Link 0 Link 1

0 closed open

1 open closed

NC open open

Jumper has three possible states: not

connected, Link 1 and Link 0. The capacitor

will charge and discharge depending on

the I/O output voltage allowing the “not

connected” state. Software should check the

“not connected” state rst by driving I/O high,

reading 1 and driving I/O low and reading 0. The

“Link 1” and “Link 0” states are read directly.

PIC

I/O

Link 0

Link 1

Figure 3-1

TIP #4 Reading DIP Switches

The input of a timer

can be used to test

which switch(s)

is closed. The

input of Timer1 is

held high with a

pull-up resistor.

Sequentially,

each switch I/O is

set to input and

Timer1 is checked

for an increment

indicating the

switch is closed.

Each bit in the DP register represents its

corresponding switch position. By setting

Timer1 to FFFFh and enabling its interrupt, an

increment will cause a rollover and generate

an interrupt. This will simplify the software by

eliminating the bit test on the TMR1L register.

Sequentially set each GPIO to an input and test

for TMR1 increment (or 0 if standard I/O pin is

used).

Figure 4-1

PIC12F6XX

GP0

GP1

GP2

GP3

GP5/T1CKI

10K

GP4Data I/O

movlw b'11111111'

movwf TRISIO

movwf DIP

movlw b'00000111'

movwf T1CON

movlw b'11111110'

movwf Mask

clrf GPIO

LOOP

clrf TMR1L

movf Mask,W

movwf TRISIO

btfsc TMR1L,0

andwf DIP,F

bsf STATUS,C

rlf Mask,F

btfsc Mask,4

goto Loop

retlw 0

Example 4-1

8-pin Flash PIC

Microcontroller Tips ‘n Tricks

TIP #5 Scanning Many Keys With

One Input

The time required to charge a capacitor

depends on resistance between V and dd

capacitor. When a button is pressed, V is dd

supplied to a different point in the resistor

ladder. The resistance between V and the dd

capacitor is reduced, which reduces the charge

time of the capacitor. A timer is used with a

comparator or changing digital input to measure

the capacitor charge time. The charge time is

used to determine which button is pressed.

Software sequence:

1. Congure GP2 to output a low voltage to

discharge capacitor through I/O resistor.

2. Congure GP2 as one comparator input and

CVref as the other.

3. Use a timer to measure when the comparator

trips. If the time measured is greater than the

maximum allowed time, then repeat;

otherwise determine which button is pressed.

When a key is pressed, the voltage divider

network changes the RC ramp rate.

Figure 5-1

PIC12F6XX

GP0

GP1

GP2

GP4

GP5

GP3

Resistors

220Ω

See AN512, “Implementing Ohmmeter/

Temperature Sensor” for code ideas.

TIP #6 Scanning Many Keys and

Wake-up From Sleep

An additional I/O can be added to wake the

part when a button is pressed. Prior to Sleep,

congure GP1 as an input with interrupt-on-

change enabled and GP2 to output high. The

pull-down resistor holds GP1 low until a button

is pressed. GP1 is then pulled high via GP2

and V generating an interrupt. After wake-up, dd

GP2 is congured to output low to discharge

the capacitor through the 220Ω resistor. GP1 is

set to output high and GP2 is set to an input to

measure the capacitor charge time.

• GP1 pin connected to key common

• Enable wake-up on port change

• Set GP1 as input and GP2 high prior to Sleep

• If key is pressed the PIC MCU wakes up, GP2

must be set low to discharge capacitor

• Set GP1 high upon wake-up to scan keystroke

Figure 6-1

VDD

100R

PIC12F6XX

GP0

GP1

GP2

GP4

GP5

GP3

220Ω

Resistors

8-pin Flash PIC

Microcontroller Tips ‘n Tricks

TIP #7 4x4 Keyboard with 1 Input

By carefully selecting the resistor values,

each button generates a unique voltage. This

voltage is measured by the A/D to determine

which button is pressed. Higher precision

resistors should be used to maximize voltage

uniqueness. The A/D will read near 0 when no

buttons are pressed.

Figure 7-1

PIC12F6XX

GP0

TIP #8 One Pin Power/Data

A single I/O can be used for both a single-

direction communication and the power source

for another microcontroller. The I/O line is held

high by the pull-up resistor connected to V . dd

The sender uses a pull-down transistor to pull

the data line low or disables the transistor to

allow the pull-up to raise it to send data to the

receiver. V is supplied to the sender through dd

the data line. The capacitor stabilizes the sender’s

V and a diode prevents the capacitor from dd

discharging through the I/O line while it is low.

Note that the V of the sender is a diode-drop dd

lower than the receiver.

Figure 8-1

Receiver

GP0 GP0

Sender

- 0.7V

8-pin Flash PIC

Microcontroller Tips ‘n Tricks

TIP #9 Decode Keys and ID Settings

Buttons and jumpers can share I/O’s by using

another I/O to select which one is read. Both

buttons and jumpers are tied to a shared

pull-down resistor. Therefore, they will read

as ‘0’ unless a button is pressed or a jumper

is connected. Each input (GP3/2/1/0) shares

a jumper and a button. To read the jumper

settings, set GP4 to output high and each

connected jumper will read as ‘1’ on its assigned

I/O or ‘0’ if it’s not connected. With GP4 output

low, a pressed button will be read as ‘1’ on its

assigned I/O and ‘0’ otherwise.

Figure 9-1

GP0

GP1

GP2

GP3

GP4

• When GP4 = 1 and no keys are pressed, read

ID setting

• When GP4 = 0, read the switch buttons

TIP #10 Generating High Voltages

Figure 10-1

PIC12F6XX

w/RC CLKOUT

CPUMP CFILTER

CLKOUT

VOUT DD DIODE max = 2 * V - 2 * V

VDD

Voltages greater than V can be generated dd

using a toggling I/O. PIC MCUs CLKOUT/OSC2

pin toggles at one quarter the frequency of

OSC1 when in external RC oscillator mode.

When OSC2 is low, the V diode is forward dd

biased and conducts current, thereby charging

C . After OSC2 is high, the other diode is pump

forward biased, moving the charge to C . filter

The result is a charge equal to twice the V dd

minus two diode drops. This can be used with a

PWM, a toggling I/O or other toggling pin.

8-pin Flash PIC

Microcontroller Tips ‘n Tricks

TIP #11 V Self Starting Circuitdd

Building on the previous topic, the same charge

pump can be used by the MCU to supply its

own V . Before the switch is pressed, V dd bat

has power and the V points are connected dd

together but unpowered. When the button is

pressed, power is supplied to V and the dd

MCUs CLKOUT (in external RC oscillator mode)

begins toggle. The voltage generated by the

charge pump turns on the FET allowing V dd

to remain powered. To power down the MCU,

execute a Sleep instruction. This allows the

MCU to switch off its power source via software.

Advantages:

• PIC MCU leakage current nearly 0

• Low cost (uses n-channel FET)

• Reliable

• No additional I/O pins required

Figure 11-1

PIC12F6XX

CLKOUT

BAT

TIP #12 Using PIC® MCU A/D For

Smart Current Limiter

Figure 12-1

PIC12F6XX

10K

AN0

SENSE

Load or Motor

• Detect current through low side sense resistor

• Optional peak lter capacitor

• Varying levels of overcurrent response can be

realized in software

By adding a resistor (R ) in series with a sense

motor, the A/D can be used to measure in-rush

current, provide current limiting, over-current

recovery or work as a smart circuit breaker. The

10K resistor limits the analog channel current

and does not violate the source impedance limit

of the A/D.

8-pin Flash PIC

Microcontroller Tips ‘n Tricks

Tip #13.1 Reading a Sensor With Higher

Accuracy – RC Timing Method

RC Timing Method:

Simple RC step response

Vc(t) = V * (1 - e -t/(RC))dd

t = -RC ln(1 - V /V )th dd

V /V is constantth dd

R2 = (t2/t1) * R1

Figure 13-1

Time

Vc(t)

t = 0 t = t1 t = t2

R1 R2

A reference resistor can be used to improve the

accuracy of an analog sensor reading. In this

diagram, the charge time of a resistor/capacitor

combination is measured using a timer and a

port input or comparator input switches from a ‘0’

to ‘1’. The R1 curve uses a reference resistor and

the R2 curve uses the sensor. The charge time

of the R1 curve is known and can be used to

calibrate the unknown sensor reading, R2. This

reduces the affects of temperature, component

tolerance and noise while reading the sensor.

TIP #13 Reading a Sensor With

Higher Accuracy

Sensors can be read directly with the A/D but in

some applications, factors such as temperature,

external component accuracy, sensor non-

linearity and/or decreasing battery voltage need

to be considered. In other applications, more

than 10 bits of accuracy are needed and a

slower sensor read is acceptable. The following

tips deal with these factors and show how to get

the most out of a PIC MCU.

13.1. RC Timing Method (with reference resistor)

13.2. Charge Balancing Method

13.3. A/D Method

8-pin Flash PIC

Microcontroller Tips ‘n Tricks

1. Set GP1 and GP2 to inputs, and GP0 to a

low output to discharge C

2. Set GP0 to an input and GP1 to a high output

3. Measure tR (GP0 changes to 1)sen

4. Repeat step 1

5. Set GP0 to an input and GP2 to a high output

6. Measure tR (GP0 changes to 1)ref

7. Use lm polypropylene capacitor

8. R = x R tRth ref sen

tRref

Figure 13-2

PIC12F629

GP0

GP1

GP2

REF

SEN

Other alternatives: voltage comparator in the

PIC12F6XX to measure capacitor voltage on

GP0.

Application Notes:

AN512, “Implementing Ohmmeter/Temperature

Sensor”

AN611, “Resistance and Capacitance Meter

Using a PIC16C622”

Here is the schematic and software ow for

using a reference resistor to improve the

accuracy of an analog sensor reading. The

reference resistor (R ) and sensor (R ) are ref sen

assigned an I/O and share a common capacitor.

GP0 is used to discharge the capacitor and

represents the capacitor voltage.

Through software, a timer is used to measure

when GP0 switches from a ‘0’ to a ‘1’ for the

sensor and reference measurements. Any

difference measured between the reference

measurement and its calibrated measurement is

used to adjust the sensor reading, resulting in a

more accurate measurement.

The comparator and comparator reference on

the PIC12F629/675 can be used instead of

a port pin for a more accurate measurement.

Polypropylene capacitors are very stable and

benecial in this type of application.

8-pin Flash PIC

Microcontroller Tips ‘n Tricks

The comparator and comparator voltage

reference (CVref) on the PIC12F629/675 are

ideal for this application.

1. GP1 average voltage = CVref

2. Time base as sampling rate

3. At the end of each time base period:

- If GP1 > CVref, then GP2 Output Low

- If GP1 < CVref, then GP2 Input mode

4. Accumulate the GP2 lows over many samples

5. Number of samples determines resolution

6. Number of GP2 lows determine effective duty

cycle of Rref

Figure 13-3

PIC12F6XX

RSEN

GP1

GP2

T1G

RREF

VDD

CVREF

COUT

Tip #13.2 Reading a Sensor With Higher

Accuracy – Charge Balancing

Method

1. Sensor charges a capacitor

2. Reference resistor discharges the capacitor

3. Modulate reference resistor to maintain

constant average charge in the capacitor

4. Use comparator to determine modulation

To improve resolution beyond 10 or 12 bits,

a technique called “Charge Balancing” can

be used. The basic concept is for the MCU

to maintain a constant voltage on a capacitor

by either allowing the charge to build through

a sensor or discharge through a reference

resistor. A timer is used to sample the

capacitor voltage on regular intervals until a

predetermined number of samples are counted.

By counting the number of times the capacitor

voltage is over an arbitrary threshold, the sensor

voltage is determined.

8-pin Flash PIC

Microcontroller Tips ‘n Tricks

TIP #14 Delta-Sigma Converter

The charge on the capacitor on GP1 is

maintained about equal to the CVref by the

MCU monitoring C and switching GP2 from out

Input mode or output low appropriately. A timer

is used to sample the C bit on a periodic out

basis. Each time GP2 is driven low, a counter is

incremented. This counter value corresponds to

the input voltage.

To minimize the affects of external component

tolerances, temperature, etc., the circuit can be

calibrated. Apply a known voltage to the input

and allow the microcontroller to count samples

until the expected result is calculated. By taking

the same number of samples for subsequent

measurements, they become calibrated

measurements.

Figure 14-1

OUT

REF

PIC12F6XX

GP1

GP2

1. GP1 average voltage = CVref

2. Time base as sampling rate

3. At the end of each time base period:

- If GP1 > CVref, then GP2 Output Low

- If GP1 < CVref, then GP2 Output High

4. Accumulate the GP2 lows over many

samples

5. Number of samples determines resolution

Tip #13.3 Reading a Sensor With Higher

Accuracy – A/D Method

NTC (Negative Temperature Coefcient)

sensors have a non-linear response to

temperature changes. As the temperature

drops, the amount the resistance changes

becomes less and less. Such sensors have

a limited useful range because the resolution

becomes smaller than the A/D resolution as the

temperature drops. By changing the voltage

divider of the R , the temperature range can sen

be expanded.

To select the higher temperature range, GP1

outputs ‘1’ and GP2 is set as an input. For

the lower range, GP2 outputs ‘1’ and GP1

is congured as an input. The lower range

will increase the amount the sensor voltage

changes as the temperature drops to allow a

larger usable sensor range.

Summary:

High range: GP1 output ‘1’ and GP2 input

Low range: GP1 input and GP2 output ‘1’

1. 10K and 100K resistors are used to set the

range

2. Vref for A/D = Vdd

3. Rth calculation is independent of Vdd

4. Count = R /(R +R ) x 255sen sen ref

5. Don’t forget to allow acquisition time for the

A/D

Figure 13-4

PIC12F675

AN0 (A/D Input)

GP1

GP2

100K

10K

SEN

8-pin Flash PIC

Microcontroller Tips ‘n Tricks

TIPS ‘N TRICKS WITH SOFTWARE

To reduce costs, designers need to make

the most of the available program memory in

MCUs. Program memory is typically a large

portion of the MCU cost. Optimizing the code

helps to avoid buying more memory than

needed. Here are some ideas that can help

reduce code size.

TIP #15 Delay Techniques

• Use GOTO “next instruction” instead of two

NOPs.

• Use CALL Rtrn as quad, 1 instruction NOP

(where “Rtrn” is the exit label from existing

subroutine).

Example 15-1

NOP

GOTO $+1

CALL Rtrn ;1 instruction, 4 cycles

Rtrn RETURN

. . .

;2 instructions, 2 cycles

;1 instruction, 2 cycles

MCUs are commonly used to interface with the

“outside world” by means of a data bus, LEDs,

buttons, latches, etc. Because the MCU runs at

a xed frequency, it will often need delay

routines to meet setup/hold times of other

devices, pause for a handshake or decrease the

data rate for a shared bus.

Longer delays are well-suited for the DECFSZ

and INCFSZ instructions where a variable is

decremented or incremented until it reaches

zero when a conditional jump is executed. For

shorter delays of a few cycles, here a few ideas

to decrease code size.

For a two-cycle delay, it is common to use

two NOP instructions which uses two program

memory locations. The same result can

be achieved by using “goto $+1”. The “$”

represents the current program counter value

in MPASM™ Assembler. When this instruction

is encountered, the MCU will jump to the next

memory location. This is what it would have

done if two NOP’s were used but since the

GOTO instruction uses two instruction cycles

to execute, a two-cycle delay was created.

This created a two-cycle delay using only one

location of program memory.

To create a four-cycle delay, add a label to an

existing RETURN instruction in the code. In

this example, the label “Rtrn” was added to the

RETURN of subroutine that already existed

somewhere in the code. When executing “CALL

Rtrn”, the MCU delays two instruction cycles

to execute the CALL and two more to execute

the RETURN. Instead of using four NOP

instructions to create a four-cycle delay, the

same result was achieved by adding a single

CALL instruction.

8-pin Flash PIC

Microcontroller Tips ‘n Tricks

TIP #18 Swap File Register with W

Example 18-1

SWAPWF MACRO REG

XORWF REG,F

XORWF REG,W

XORWF REG,F

ENDM

The following macro swaps the contents of W

and REG without using a second register.

Needs: 0 TEMP registers

3 Instructions

3 TCY

An efcient way of swapping the contents of a

XORWF instructions. It requires no temporary

registers and three instructions. Here’s an

example:

W REG Instruction

10101100 01011100 XORWF REG,F

10101100 11110000 XORWF REG,W

01011100 11110000 XORWF REG,F

01011100 10101100 Result

TIP #19 Bit Shifting Using Carry Bit

Rotate a byte through carry without using RAM

variable for loop count:

• Easily adapted to serial interface transmit

routines.

• Carry bit is cleared (except last cycle) and the

cycle repeats until the zero bit sets indicating

the end.

Example 19-1

bsf

rlf

bcf

btfsc

bsf

bcf

rlf

movf

btfss

goto

LIST P=PIC12f629

INCLUDE P12f629.INC

buffer

STATUS,C

buffer,f

GPIO,Dout

STATUS,C

GPIO,Dout

STATUS,C

buffer,f

STATUS,Z

Send_Loop

equ 0x20

;Set 'end of loop' flag

;Place first bit into C

;precondition output

;Check data 0 or 1 ?

;Clear data in C

;Place next bit into C

;Force Z bit

;Exit?

PIC

Microcontroller Low Power Tips ‘n Tricks

Table Of Contents

GENERAL LOW POWER TIPS ‘N TRICKS

TIP #1 Switching Off External Circuits/

Duty Cycle .......................................... 2-2

TIP #2 Power Budgeting ................................ 2-3

TIP #3 Conguring Port Pins ......................... 2-4

TIP #4 Use High-Value Pull-Up Resistors ...... 2-4

TIP #5 Reduce Operating Voltage ................. 2-4

TIP #6 Use an External Source for

CPU Core Voltage .............................. 2-5

TIP #7 Battery Backup for PIC MCUs ........... 2-6

DYNAMIC OPERATION TIPS ‘N TRICKS

TIP #8 Enhanced PIC16 Mid-Range Core ..... 2-6

TIP #9 Two-Speed Start-Up ........................... 2-7

TIP #10 Clock Switching .................................. 2-7

TIP #11 Use Internal RC Oscillators ................ 2-7

TIP #12 Internal Oscillator Calibration ............. 2-8

TIP #13 Idle and Doze Modes ......................... 2-8

TIP #14 Use and Idle Mode ...................... 2-9NOP

TIP #15 Peripheral Module Disable

(PMD) Bits .......................................... 2-9

STATIC POWER REDUCTION TIPS ‘N TRICKS

TIP #16 Deep Sleep Mode ............................... 2-10

TIP #17 Extended WDT and Deep

Sleep WDT ......................................... 2-10

TIP #18 Low Power Timer1 Oscillator

and RTCC........................................... 2-10

TIP #19 Low Power Timer1 Oscillator Layout 2-11 ..

TIP #20 Use LVD to Detect Low Battery .......... 2-11

TIP #21 Use Peripheral FIFO and DMA ........... 2-11

TIP #22 Ultra Low-Power

Wake-Up Peripheral ........................... 2-12

TIPS ‘N TRICKS INTRODUCTION

Microchip continues to provide innovative

products that are smaller, faster, easier to

use and more reliable. The Flash-based PIC ®

microcontrollers (MCUs) are used in an wide

range of everyday products, from smoke

detectors, hospital ID tags and pet containment

systems, to industrial, automotive and medical

products.

PIC MCUs featuring nanoWatt technology

implement a variety of important features which

have become standard in PIC microcontrollers.

Since the release of nanoWatt technology,

changes in MCU process technology and

improvements in performance have resulted in

new requirements for lower power. PIC MCUs

with nanoWatt eXtreme Low Power (nanoWatt

XLP™) improve upon the original nanoWatt

technology by dramatically reducing static

power consumption and providing new exibility

for dynamic power management.

The following series of Tips n’ Tricks can be

applied to many applications to make the most

of PIC MCU nanoWatt and nanoWatt XLP

devices.

GENERAL LOW POWER TIPS ‘N

TRICKS

The following tips can be used with all PIC

MCUs to reduce the power consumption of

almost any application.

CHAPTER 2

PIC ® Microcontroller Low Power

Tips ‘n Tricks

PIC

Microcontroller Low Power Tips ‘n Tricks

TIP #1 Switching Off External

Circuits/Duty Cycle

All the low power modes in the world won’t help

your application if you are unable to control

the power used by circuits external to the

microprocessor. Lighting an LED is equivalent

to running most PIC MCUs at 5V-20 MHz.

When you are designing your circuitry, decide

what physical modes or states are required and

partition the electronics to shutdown unneeded

circuitry.

Figure: 1-1

PIC16F819

32.768 kHz

0.1 Fµ

33 pF

10k

22 pF

100k

0.1 Fµ

3.3V

Serial EEPROM

GND

VCC

SCL

SDA

RB0/INT

RB1

RB2

RB3

RB4

RB5

RB6

RB7

VDD

RA0

RA1

RA2

RA3

RA4/ CKITO

OSC1/CLKIN

OSC2/CLKOUT

VSS

MCLR

The system shown above is very simple

and clearly has all the parts identied in the

requirements. Unfortunately, it has a few

problems in that the EEPROM, the sensor, and

its bias circuit, are energized all the time. To

get the minimum current draw for this design,

it would be advantageous to shutdown these

circuits when they are not required.

Figure: 1-2

Example:

The application is a long duration data recorder.

It has a sensor, an EEPROM, a battery and a

microprocessor. Every two seconds, it must

take a sensor reading, scale the sensor data,

store the scaled data in EEPROM and wait for

the next sensor reading.

In Figure 1-2, I/O pins are used to power the

EEPROM and the sensor. Many PIC MCU

devices can source up to 20 mA of current

from each I/O, so there is no need to provide

additional components to switch the power.

If more current than can be sourced by the PIC

MCU is required, the PIC MCU can instead

enable and disable a MOSFET to power

the circuit. Refer to the data sheet for drive

capabilities for a specic device.

Serial EEPROM

GND

VCC

SCL

SDA

0.1 Fµ

33 pF

32.768 kHz

33 pF

10k

PIC16F819

RB0/INT

RB1

RB2

RB3

RB4

RB5

RB6

RB7

RA0

RA1

RA2

RA3

RA4/ CKITO

OSC1/CLKIN

OSC2/CLKOUT

MCLR

100k

22 pF

100k

0.1 Fµ

3.3V

PIC

Microcontroller Low Power Tips ‘n Tricks

TIP #2 Power Budgeting

Power budgeting is a technique that is critical to

predicting current consumption and battery life.

Power budgeting is performed by calculating

the total charge for each mode of operation

of an application by multiplying that mode’s

current consumption by the time in the mode

for a single application loop. The charge for

each mode is added, then averaged over the

total loop time to get average current. Table 1

calculates a power budget using the application

from Figure 2 in Tip #1 using a typical nanoWatt

XLP device.

Mode

Time

Mode

(mS)

Current (mA) Charge

Current *

Time

(mA * Sec)

Device

Mode

Total

Sleep

MCU Sleep

Sensor Off

EEPROM Off

1989 5.00E-05 9.95E-05

0.00005

Initialize

MCU Sleep

Sensor On

EEPROM Off

1 1.66E-02 1.66E-05

0.00005

0.0165

Sample Sensor

MCU Run

Sensor On

EEPROM Off

1 6.45E-02 6.45E-05

0.048

0.0165

Scaling

MCU Run

Sensor Off

EEPROM Off

1 4.80E-02 4.80E-05

0.048

Storing

MCU Run

Sensor Off

EEPROM On

8 1.05E+00 8.38E-03

0.048

Total 2000 — — 8.61E-03

Average Current

= 8.61e-3

2000e-3

mA*Sec

Sec

= 0.0043 mA

Peak Current 1.05 mA

Computing Battery Life

Using the average current from the calculated

power budget, it is possible to determine

how long a battery will be able to power the

application. Table 2 shows lifetimes for typical

battery types using the average power from

Table 1.

Battery Capacity

(mAh)

Life

Hours Days Months Years

CR1212 18 4180 174 5.8 .48

CR1620 75 17417 726 24.2 1.99

CR2032 220 51089 2129 71.0 5.83

Alkaline AAA 1250 290276 12095 403.2 33.14

Alkaline AA 2890 671118 27963 932.1 76.61

Li-ion* 850 197388 8224 274.1 22.53

NOTE: Calculations are based on average current draw only and

do not include battery self-discharge.

*Varies by size; value used is typical.

After completing a power budget, it is very easy

to determine the battery size required to meet

the application requirements. If too much power

is consumed, it is simple to determine where

additional effort needs to be placed to reduce

the power consumption.

PIC

Microcontroller Low Power Tips ‘n Tricks

TIP #3 Conguring Port Pins

All PIC MCUs have bidirectional I/O pins. Some

of these pins have analog input capabilities. It

is very important to pay attention to the signals

applied to these pins so the least amount of

power will be consumed.

Unused Port Pins

If a port pin is unused, it may be left

unconnected but congured as an output pin

driving to either state (high or low), or it may

be congured as an input with an external

resistor (about 10 kΩ) pulling it to Vdd or V . ss

If congured as an input, only the pin input

leakage current will be drawn through the

pin (the same current would ow if the pin

was connected directly to V or V ). Both dd ss

options allow the pin to be used later for either

input or output without signicant hardware

modications.

Digital Inputs

A digital input pin consumes the least amount

of power when the input voltage is near V dd

or V . If the input voltage is near the midpoint ss

between V and V , the transistors inside the dd ss

digital input buffer are biased in a linear region

and they will consume a signicant amount

of current. If such a pin can be congured as

an analog input, the digital buffer is turned off,

reducing both the pin current as well as the total

controller current.

Analog Inputs

Analog inputs have a very high-impedance

so they consume very little current. They

will consume less current than a digital input

if the applied voltage would normally be

centered between V and V . Sometimes it dd ss

is appropriate and possible to congure digital

inputs as analog inputs when the digital input

must go to a low power state.

Digital Outputs

There is no additional current consumed by a

digital output pin other than the current going

through the pin to power the external circuit.

Pay close attention to the external circuits to

minimize their current consumption.

TIP #4 Use High-Value Pull-Up

Resistors

It is more power efcient to use larger pull-up

resistors on I/O pins such as MCLR, I 2C™

signals, switches and for resistor dividers. For

example, a typical I 2C pull-up is 4.7k. However,

when the I 2C is transmitting and pulling a line

low, this consumes nearly 700 uA of current for

each bus at 3.3V. By increasing the size of the

I2C pull-ups to 10k, this current can be halved.

The tradeoff is a lower maximum I 2C bus

speed, but this can be a worthwhile trade in for

many low power applications. This technique is

especially useful in cases where the pull-up can

be increased to a very high resistance such as

100k or 1M.

TIP #5 Reduce Operating Voltage

Reducing the operating voltage of the device,

V , is a useful step to reduce the overall dd

power consumption. When running, power

consumption is mainly inuenced by the clock

speed. When sleeping, the most signicant

factor is leakage in the transistors. At lower

voltages, less charge is required to switch the

system clocks and transistors leak less current.

It is important to pay attention to how reducing

the operating voltage reduces the maximum

allowed operating frequency. Select the

optimum voltage that allows the application

to run at its maximum speed. Refer to the

device data sheet for the maximum operating

frequency of the device at the given voltage.

PIC

Microcontroller Low Power Tips ‘n Tricks

TIP #6 Use an External Source for

CPU Core Voltage

Some PIC MCUs such as “J” type devices (ex.

PIC18F87J90 or PIC24FJ64GA004) use sepa-

rate power for CPU core. These devices have

an internal voltage regulator that can be used to

provide the core voltage. Alternatively, the core

voltage can be provided externally by disabling

the internal regulator. In some cases, it is more

power efcient to use an external source for

the core. This is because the internal regula-

tor powers the core at the nominal voltage that

allows full speed operation. However, if an

application doesn’t require full speed, it is ben-

ecial to use lower voltage to power the core.

Disabling the internal regulator also turns off the

BOR and LVD circuits, which saves power as

well. The following examples show two different

battery powered applications where it can be

benecial to disable the internal regulator.

Example 1: Constant Voltage Source

When using a regulated power source or a

battery with a at discharge curve, such as a

lithium coin cell, the regulator can be disabled

and the core powered directly from the battery

through a diode. The diode provides the

voltage drop necessary to power the core at the

correct voltage. It may be necessary to use a

zener diode with a higher forward voltage for

applications using sleep mode, as the current

consumed in sleep is too low to cause the

full forward voltage drop which can result in

applying a voltage too high for the core.

Figure 6-1:

Example 2: Non-Constant Voltage Source

If the source for V is not constant, a regulator dd

will be required. It can be benecial to use an

external low quiescent current regulator, which

can be selected to provide lower voltage to the

core than the internal regulator. Additionally,

devices such as the MCP1700, which

consumes 1 uA quiescent current while asleep,

require less power than the internal regulator.

Figure 6-2:

PIC

Microcontroller Low Power Tips ‘n Tricks

TIP #7 Battery Backup for PIC MCUs

For an application that can operate from either

an external supply or a battery backup, it is

necessary to be able to switch from one to

the other without user intervention. This can

be accomplished with battery backup ICs, but

it is also possible to implement with a simple

diode OR circuit, shown in Figure 7-1. Diode D1

prevents current from owing into the battery

from VEXT when the external power is sup-

plied. D2 prevents current from owing into any

external components from the battery if VEXT

is removed. As long as the external source is

present and higher voltage than the battery,

no current from the battery will be used. When

VEXT is removed and the voltage drops below

VBAT, the battery will start powering the MCU.

Low forward voltage Schottky diodes can be

used in order to minimize the voltage dropout

from the diodes. Additionally, inputs can be ref-

erenced to VEXT and VBAT in order to monitor

the voltage levels of the battery and the exter-

nal supply. This allows the micro to enter lower

power modes when the supply is removed or

the battery is running low. In order to avoid

glitches on V caused by the diode turn-on dd

delay when switching supplies, ensure enough

decoupling capacitance is used on V (C1).dd

Figure 7-1:

Dynamic Operation Tips n’ Tricks

The following tips and tricks apply to methods

of improving the dynamic operating current

consumption of an application. This allows

an application to get processing done quicker

which enables it to sleep more and will help

reduce the current consumed while processing.

TIP #8 Enhanced PIC16 Mid-Range

Core

The Enhanced PIC16 mid-range core has a few

features to assist in low power. New instructions

allow many applications to execute in less

time. This allows the application to spend more

time asleep and less time processing and

can provide considerable power savings. It is

important not to overlook these new instructions

when designing with devices that contain the

new core. The Timer1 oscillator and WDT have

also been improved, now meeting nanoWatt

XLP requirements and drawing much less

current than in previous devices.















PIC

Microcontroller Low Power Tips ‘n Tricks

TIP #9 Two-Speed Start-Up

Two-speed startup is a useful feature on some

nanoWatt and all nanoWatt XLP devices which

helps reduce power consumption by allowing

the device to wake up and return to sleep

faster. Using the internal oscillator, the user can

execute code while waiting for the Oscillator

Start-up (OST) timer to expire (LP, XT or HS

modes). This feature (called “Two-Speed Start-

up”) is enabled using the IESO conguration

bit. A Two-Speed Start-up will clock the device

from an internal RC oscillator until the OST has

expired. Switching to a faster internal oscillator

frequency during start-up is also possible using

the OSCCON register. The example below

shows several stages on how this can be

achieved. The number of frequency changes

is dependent upon the designer’s discretion.

Assume a 20 MHz crystal (HS Mode) in the

PIC16F example below.

Example:

Tcy

(Instruction Time) Instruction

ORG 0x05 ;Reset vector

125 s @ 32 kHz STATUS,RP0 ;bank1m BSF

125 s @ 32 kHz OSCCON,IRCF2 ;switch to 1 MHzm BSF

4 s @ 1 MHz OSCCON,IRCF1 ;switch to 4 MHzm BSF

1 s @ 4 MHz OSCCON,IRCF0 ;switch to 8 MHzm BSF

500 ns application code

… ….

.. …

(eventually OST expires, 20 MHz crystal clocks the device)

200 ns application code

… ….

.. …

TIP #10 Clock Switching

Some nanoWatt devices and all nanoWatt XLP

devices have multiple internal and external

clock sources, as well as logic to allow switching

between the available clock sources as the

main system clock. This allows for signicant

power savings by choosing different clocks

for different portions of code. For example, an

application can use the slower internal oscillator

when executing non-critical code and then

switch to a fast high-accuracy oscillator for time

or frequency sensitive code. Clock switching

allows much more exible applications than

being stuck with a single clock source. Clock

switching sequences vary by device family, so

refer to device data sheets or Family Reference

Manuals for the specic clock switching

sequences.

TIP #11 Use Internal RC Oscillators

If frequency precision better than ±5% is not

required, it is best to utilize the internal RC

oscillators inside all nanoWatt and nanoWatt

XLP devices. The internal RC oscillators have

better frequency stability than external RC

oscillators, and consume less power than

external crystal oscillators. Additionally, the

internal clock can be congured for many

frequency ranges using the internal PLL module

to increase frequency and the postscaler to

reduce it. All these options can be congured in

rmware.

PIC

Microcontroller Low Power Tips ‘n Tricks

TIP #12 Internal Oscillator Calibration

An internal RC oscillator calibrated from the

factory may require further calibration as the

temperature or V change. Timer1/SOSC can dd

be used to calibrate the internal oscillator by

connecting a 32.768 kHz clock crystal. Refer

to AN244, “ ” Internal RC Oscillator Calibration

for the complete application details. Calibrating

the internal oscillator can help save power by

allowing for use of the internal RC oscillator

in applications which normally require higher

accuracy crystals

Figure 12-1: Timer1 Used to Calibrate an

Internal Oscillator

PIC16F818/819

T1OSI

T1OSO

33 pF

XTAL

32.768 kHz

The calibration is based on the measured

frequency of the internal RC oscillator. For

example, if the frequency selected is 4 MHz,

we know that the instruction time is 1 µs

(F /4) and Timer1 has a period of 30.5 µs osc

(1/32.768 kHz). This means within one Timer1

period, the core can execute 30.5 instructions.

If the Timer1 registers are preloaded with a

known value, we can calculate the number of

instructions that will be executed upon a Timer1

overow.

This calculated number is then compared

against the number of instructions executed by

the core. With the result, we can determine if

re-calibration is necessary, and if the frequency

must be increased or decreased. Tuning uses

the OSCTUNE register, which has a ±12%

tuning range in 0.8% steps.

TIP #13 Idle and Doze Modes

nanoWatt and nanoWatt XLP devices have

an Idle mode where the clock to the CPU is

disconnected and only the peripherals are

clocked. In PIC16 and PIC18 devices, Idle

mode can be entered by setting the Idle bit in

the OSCON register to ‘ ’ and executing the 1

SLEEP instruction. In PIC24, dsPIC ® DSCs,

and PIC32 devices, Idle mode can be entered

by executing the instruction “ ”. Idle PWRSAV #1

mode is best used whenever the CPU needs to

wait for an event from a peripheral that cannot

operate in Sleep mode. Idle mode can reduce

power consumption by as much as 96% in

many devices.

Doze mode is another low power mode

available in PIC24, dsPIC DSCs, and PIC32

devices. In Doze mode, the system clock to

the CPU is postscaled so that the CPU runs at

a lower speed than the peripherals. If the CPU

is not tasked heavily and peripherals need to

run at high speed, then Doze mode can be

used to scale down the CPU clock to a slower

frequency. The CPU clock can be scaled down

from 1:1 to 1:128. Doze mode is best used in

similar situations to Idle mode, when peripheral

operation is critical, but the CPU only requires

minimal functionality.

PIC

Microcontroller Low Power Tips ‘n Tricks

Static Power Reduction Tips n’ Tricks

The following tips and tricks will help reduce

the power consumption of a device while it is

asleep. These tips allow an application to stay

asleep longer and to consume less current

while sleeping.

TIP #16 Deep Sleep Mode

In Deep Sleep mode, the CPU and all

peripherals except RTCC, DSWDT and

LCD (on LCD devices) are not powered.

Additionally, Deep Sleep powers down the

Flash, SRAM, and voltage supervisory circuits.

This allows Deep Sleep mode to have lower

power consumption than any other operating

mode. Typical Deep Sleep current is less than

50 nA on most devices. Four bytes of data are

retained in the DSGPRx registers that can be

used to save some critical data required for the

application. While in Deep Sleep mode, the

states of I/O pins and 32 kHz crystal oscillator

(Timer1/SOSC) are maintained so that Deep

Sleep mode does not interrupt the operation of

the application. The RTCC interrupt, Ultra Low

Power Wake-up, DSWDT time-out, External

Interrupt 0 (INT0), MCLR or POR can wake-up

the device from Deep Sleep. Upon wake-up the

device resumes operation at the reset vector.

Deep Sleep allows for the lowest possible

static power in a device. The trade-off is that

the rmware must re-initialize after wake-

up. Therefore, Deep Sleep is best used in

applications that require long battery life and

have long sleep times. Refer to the device

datasheets and Family Reference Manuals for

more information on Deep Sleep and how it is

used.

TIP #17 Extended WDT and Deep

Sleep WDT

A commonly used source to wake-up from

Sleep or Deep Sleep is the Watchdog Timer

(WDT) or Deep Sleep Watchdog Timer

(DSWDT). The longer the PIC MCU stays

in Sleep or Deep Sleep, the less power

consumed. Therefore, it is appropriate to use

as long a timeout period for the WDT as the

application will allow.

The WDT runs in all modes except for Deep

Sleep. In Deep Sleep, the DSWDT is used

instead. The DSWDT uses less current and

has a longer timeout period than the WDT. The

timeout period for the WDT varies by device,

but typically can vary from a few milliseconds to

up to 2 minutes. The DSWDT time-out period

can be programmed from 2.1ms to 25.7days

TIP #18 Low Power Timer1 Oscillator

and RTCC

nanoWatt XLP microcontrollers all have a

robust Timer1 oscillator (SOSC on PIC24)

which draws less than 800 nA. nanoWatt

technology devices offer a low power Timer1

oscillator which draws 2-3 uA. Some devices

offer a selectable oscillator which can be used

in either a low-power or high-drive strength

mode to suit both low power or higher noise

applications. The Timer1 counter and oscillator

can be used to generate interrupts for periodic

wakes from Sleep and other power managed

modes, and can be used as the basis for a real-

time clock. Timer1/SOSC wake-up options vary

by device. Many nanoWatt XLP devices have a

built-in hardware Real-Time Clock and Calendar

(RTCC), which can be congured for wake-up

periods from 1 second to many years.

Some nanoWatt devices and all nanoWatt

XLP devices can also use the Timer1/SOSC

oscillator as the system clock source in place

of the main oscillator on the OSC1/OSC2 pins.

By reducing execution speed, total current

consumption can be reduced.

PIC

Microcontroller Low Power Tips ‘n Tricks

TIP #19 Low Power Timer1 Oscillator

Layout

Applications requiring very low power Timer1/

SOSC oscillators on nanoWatt and nanoWatt

XLP devices must take PCB layout into

consideration. The very low power Timer1/

SOSC oscillators on nanoWatt and nanoWatt

XLP devices consume very little current, and

this sometimes makes the oscillator circuit

sensitive to neighboring circuits. The oscillator

circuit (crystal and capacitors) should be located

as close as possible to the microcontroller.

No circuits should be passing through the

oscillator circuit boundaries. If it is unavoidable

to have high-speed circuits near the oscillator

circuit, a guard ring should be placed around the

oscillator circuit and microcontroller pins similar

to the gure below. Placing a ground plane

under the oscillator components also helps to

prevent interaction with high speed circuits.

Figure 19-1: Guard Ring Around Oscillator

Circuit and MCU Pins

VSS

OSC2

OSC1

RB6

RB7

RB5

TIP #20 Use LVD to Detect Low

Battery

The Low Voltage Detect (LVD) interrupt present

in many PIC MCUs is critical in battery based

systems. It is necessary for two reasons.

First, many devices cannot run full speed at

the minimum operating voltage. In this case,

the LVD interrupt indicates when the battery

voltage is dropping so that the CPU clock can

be slowed down to an appropriate speed,

preventing code misexecution. Second, it allows

the MCU to detect when the battery is nearing

the end of its life, so that a low battery indication

can be provided and a lower power state can

be entered to maximize battery lifetime. The

LVD allows these functions to be implemented

without requiring the use of extra analog

channels to measure the battery level.

TIP #21 Use Peripheral FIFO and

DMA

Some devices have peripherals with DMA or

FIFO buffers. These features are not just useful

to improve performance; they can also be used

to reduce power. Peripherals with just one

buffer register require the CPU to stay operating

in order to read from the buffer so it doesn’t

overow. However, with a FIFO or DMA, the

CPU can go to sleep or idle until the FIFO lls or

DMA transfer completes. This allows the device

to consume a lot less average current over the

life of the application.

PIC

Microcontroller Low Power Tips ‘n Tricks

TIP #22 Ultra Low-Power Wake-Up

Peripheral

Newer devices have a modication to PORTA

that creates an Ultra Low-Power Wake-Up

(ULPWU) peripheral. A small current sink and

a comparator have been added that allows

an external capacitor to be used as a wake-

up timer. This feature provides a low-power

periodic wake-up source which is dependent on

the discharge time of the external RC circuit.

Figure 22-1: Ultra Low-Power Wake-Up

Peripheral

VREF

Pin Wake-on-Change

Interrupt

If the accuracy of the Watchdog Timer is not

required, this peripheral can save a lot of

current.

Visit the low power design center at:

www.microchip.com/lowpower for

additional design resources.

PIC

Microcontroller CCP and ECCP Tips ‘n Tricks

Table Of Contents

CAPTURE TIPS ‘N TRICKS

TIP #1 Measuring the Period of a

Square Wave ...................................... 3-3

TIP #2 Measuring the Period of a

Square Wave with Averaging ............. 3-3

TIP #3 Measuring Pulse Width ...................... 3-4

TIP #4 Measuring Duty Cycle ........................ 3-4

TIP #5 Measuring RPM Using an Encoder .... 3-5

TIP #6 Measuring the Period of an

Analog Signal ..................................... 3-6

COMPARE TIPS ‘N TRICKS

TIP #7 Periodic Interrupts .............................. 3-8

TIP #8 Modulation Formats ............................ 3-9

TIP #9 Generating the Time Tick

for a RTOS ......................................... 3-10

TIP #10 16-Bit Resolution PWM ...................... 3-10

TIP #11 Sequential ADC Reader ..................... 3-11

TIP #12 Repetitive Phase Shifted Sampling .... 3-12

PWM TIPS ‘N TRICKS

TIP #13 Deciding on PWM Frequency ............. 3-14

TIP #14 Unidirectional Brushed DC

Motor Control Using CCP ................... 3-14

TIP #15 Bidirectional Brushed DC

Motor Control Using ECCP................. 3-15

TIP #16 Generating an Analog Output ............. 3-16

TIP #17 Boost Power Supply ........................... 3-17

TIP #18 Varying LED Intensity .........................3-18

TIP #19 Generating X-10 Carrier Frequency ... 3-18

COMBINATION CAPTURE AND COMPARE TIPS

TIP #20 RS-232 Auto-baud .............................. 3-19

TIP #21 Dual-Slope Analog-to-Digital

Converter ............................................ 3-21

TIPS ‘N TRICKS INTRODUCTION

Microchip continues to provide innovative

products that are smaller, faster, easier-to-use

and more reliable. PIC® microcontrollers (MCUs)

are used in a wide range of everyday products,

from washing machines, garage door openers

and television remotes to industrial, automotive

and medical products.

The Capture, Compare and PWM (CCP)

modules that are found on many of Microchip’s

microcontrollers are used primarily for the

measurement and control of time-based pulse

signals. The Enhanced CCP (ECCP), available

on some of Microchip’s devices, differs from

the regular CCP module in that it provides

enhanced PWM functionality – namely,

full-bridge and half-bridge support,

programmable dead-band delay and enhanced

PWM auto-shutdown. The ECCP and CCP

modules are capable of performing a wide

variety of tasks. This document will describe

some of the basic guidelines to follow when

using these modules in each mode, as well as

give suggestions for practical applications.

CHAPTER 3

PIC® Microcontroller CCP and ECCP

Tips ‘n Tricks

PIC

Microcontroller CCP and ECCP Tips ‘n Tricks

ECCP/CCP Register Listing

Capture

Mode

Compare

Mode PWM Mode

CCPxCON Select mode Select mode Select mode,

LSB of duty

cycle

CCPRxL Timer1

capture

(LSB)

Timer1

compare

(LSB)

MSB of duty

cycle

CCPRxH Timer1

capture

(MSB)

Timer1

compare

(MSB)

N/A

TRISx Set CCPx

pin to input

Set CCPx pin

to output

Set CCPx pin(s)

to output(s)

T1CON Timer1 on,

prescaler

Timer1 on,

prescaler

N/A

T2CON N/A N/A Timer2 on,

prescaler

PR2 N/A N/A Timer2 period

PIE1 Timer1

interrupt

enable

Timer1

interrupt

enable

Timer2 interrupt

enable

PIR1 Timer1

interrupt ag

Timer1

interrupt ag

Timer2 interrupt

ag

INTCON Global/

peripheral

interrupt

enable

Global/

peripheral

interrupt

enable

Global/

peripheral

interrupt enable

PWM1CON(1) N/A N/A Set dead band,

auto-restart

control

ECCPAS(1) N/A N/A Auto-shutdown

control

Note 1: Only on ECCP module.

CAPTURE TIPS ‘N TRICKS

In Capture mode, the 16-bit value of Timer1

is captured in CCPRxH:CCPRxL when an

event occurs on pin CCPx. An event is dened

as one of the following and is congured by

CCPxCON<3:0>:

• Every falling edge

• Every rising edge

• Every 4th rising edge

• Every 16th rising edge

“When Would I Use Capture Mode?”

Capture mode is used to measure the length of

time elapsed between two events. An event, in

general, is either the rising or falling edge of a

signal (see Figure 1 “Dening Events”).

An example of an application where Capture

mode is useful is reading an accelerometer.

Accelerometers typically vary the duty

cycle of a square wave in proportion to the

acceleration acting on a system. By conguring

the CCP module in Capture mode, the PIC

microcontroller can measure the duty cycle of

the accelerometer with little intervention on the

part of the microcontroller rmware. Tip #4 goes

into more detail about measuring duty cycle by

conguring the CCP module in Capture mode.

Figure 1: Dening Events

Volts

Event: Rising Edge

Event: Falling Edge

Time

PIC

Microcontroller CCP and ECCP Tips ‘n Tricks

TIP #1 Measuring the Period of a

Square Wave

Figure 1-1: Period

t1 t2

1. Congure control bits CCPxM3:CCPxM0

(CCPxCON<3:0>) to capture every rising

edge of the waveform.

2. Congure the Timer1 prescaler so Timer1

with run Tmax(1) without overowing.

3. Enable the CCP interrupt (CCPxIE bit).

4. When a CCP interrupt occurs:

a) Subtract saved captured time (t1) from

captured time (t2) and store (use Timer1

interrupt ag as overow indicator).

b) Save captured time (t2).

c) Clear Timer1 ag if set.

The result obtained in step 4.a is the period (T).

Note 1: Tmax is the maximum pulse period

that will occur.

TIP #2 Measuring the Period of a

Square Wave with Averaging

Figure 2-1: Period Measurement

t1 t2

16 x T

1. Congure control bits CCPxM3:CCPxM0

(CCPxCON<3:0>) to capture every 16th

rising edge of the waveform.

2. Congure the Timer1 prescaler so Timer1 will

run 16 Tmax(1) without overowing.

3. Enable the CCP interrupt (CCPxIE bit).

4. When a CCP interrupt occurs:

a) Subtract saved captured time (t1) from

captured time (t2) and store (use Timer1

interrupt ag as overow indicator).

b) Save captured time (t2).

c) Clear Timer1 ag if set.

d) Shift value obtained in step 4.a right four

times to divide by 16 – this result is the

period (T).

Note 1: Tmax is the maximum pulse period

that will occur.

The following are the advantages of this

method as opposed to measuring the periods

individually.

• Fewer CCP interrupts to disrupt program ow

• Averaging provides excellent noise immunity

PIC

Microcontroller CCP and ECCP Tips ‘n Tricks

TIP #3 Measuring Pulse Width

Figure 3-1: Pulse Width

t1 t2

1. Congure control bits CCPxM3:CCPxM0

(CCPxCON<3:0>) to capture every rising

edge of the waveform.

2. Congure Timer1 prescaler so that Timer1

will run Wmax without overowing.

3. Enable the CCP interrupt (CCPxIE bit).

4. When CCP interrupt occurs, save the

captured timer value (t1) and recongure

control bits to capture every falling edge.

5. When CCP interrupt occurs again, subtract

saved value (t1) from current captured value

(t2) – this result is the pulse width (W).

6. Recongure control bits to capture the next

rising edge and start process all over again

(repeat steps 3 through 6).

TIP #4 Measuring Duty Cycle

Figure 4-1: Duty Cycle

t1 t2 t3

The duty cycle of a waveform is the ratio

between the width of a pulse (W) and the

period (T). Acceleration sensors, for example,

vary the duty cycle of their outputs based on

the acceleration acting on a system. The CCP

module, congured in Capture mode, can be

used to measure the duty cycle of these types

of sensors. Here’s how:

1. Congure control bits CCPxM3:CCPxM0

(CCPxCON<3:0>) to capture every rising

edge of the waveform.

2. Congure Timer1 prescaler so that Timer1

will run Tmax(1) without overowing.

3. Enable the CCP interrupt (CCPxIE bit).

4. When CCP interrupt occurs, save the

captured timer value (t1) and recongure

control bits to capture every falling edge.

Note 1: Tmax is the maximum pulse period

that will occur.

5. When the CCP interrupt occurs again,

subtract saved value (t1) from current

captured value (t2) – this result is the pulse

width (W).

6. Recongure control bits to capture the next

rising edge.

7. When the CCP interrupt occurs, subtract

saved value (t1) from the current captured

value (t3) – this is the period (T) of the

waveform.

8. Divide T by W – this result is the Duty Cycle.

9. Repeat steps 4 through 8.

PIC

Microcontroller CCP and ECCP Tips ‘n Tricks

TIP #5 Measuring RPM Using an

Encoder

Revolutions Per Minute (RPM), or how fast

something turns, can be sensed in a variety of

ways. Two of the most common sensors used

to determine RPM are optical encoders and

Hall effect sensors. Optical encoders detect the

presence of light shining through a slotted wheel

mounted to a turning shaft (see Figure 5-1.)

As the shaft turns, the slots in the wheel pass

by the eye of the optical encoder. Typically, an

infrared source on the other side of the wheel

emits light that is seen by the optical encoder

through slots in the wheel. Hall effect sensors

work by sensing the position of the magnets in

an electric motor, or by sensing a permanent

magnet mounted to a rotating object (see Figure

5-2). These sensors output one or more pulses

per revolution (depending on the sensor).

Figure 5-1: Optical Encoder

Figure 5-2: Hall Effect Sensor

Slotted

Wheel

IR LED IR Sensor

Front View ie View

Wheel

Magnet

Hall effect

Sensor

In Figure 5-3 and Figure 5-4, the waveform

is high when light is passing through a slot in

the encoder wheel and shining on the optical

sensor. In the case of a Hall effect sensor, the

high corresponds to the time that the magnet is

in front of the sensor. These gures show the

difference in the waveforms for varying RPMs.

Notice that as RPM increases, the period (T)

and pulse width (W) becomes smaller. Both

period and pulse width are proportional to RPM.

However, since the period is the greater of the

two intervals, it is good practice to measure the

period so that the RPM reading from the sensor

will have the best resolution. See Tip #1 for

measuring period. The technique for measuring

period with averaging described in Tip #2 is

useful for measuring high RPMs.

Figure 5-3: Low RPM

Figure 5-4: High RPM

PIC

Microcontroller CCP and ECCP Tips ‘n Tricks

TIP #6 Measuring the Period of an

Analog Signal

Microcontrollers with on-board Analog

Comparator module(s), in addition to a CCP

(or ECCP) module, can easily be congured to

measure the period of an analog signal.

Figure 6-1 shows an example circuit using the

peripherals of the PIC16F684.

Figure 6-1: Circuit

PIC16F684

THR

SENSE

Comparator

(on-board PIC16F684)

OUT

Analog

Input

REF

CCP1

R3 and R4 set the threshold voltage for the

comparator. When the analog input reaches

the threshold voltage, Vout will toggle from

low to high. R1 and R2 provide hysteresis to

insure that small changes in the analog input

won’t cause jitter in the circuit. Figure 6-2

demonstrates the effect of hysteresis on the

input. Look specically at what Vsense does

when the analog input reaches the threshold

voltage.

Figure 6-2: Signal Comparison

The CCP module, congured in Capture mode,

can time the length between the rising edges of

the comparator output (Vout.) This is the period

of the analog input, provided the analog signal

reaches V during every period.thr

time

THR

SENSE

OUT

nalog

Input

THR

time

PIC

Microcontroller CCP and ECCP Tips ‘n Tricks

COMPARE TIPS ‘N TRICKS

In Compare mode, the 16-bit CCPRx register

value is constantly compared against the TMR1

CCPx pin is:

• Driven high

• Driven low

• Remains unchanged, or

• Toggles based on the module’s conguration

The action on the pin is determined by control

bits CCPxM3:CCPxM0 (CCPxCON<3:0>).

A CCP interrupt is generated when a match

occurs.

Special Event Trigger

Timer1 is normally not cleared during a CCP

interrupt when the CCP module is congured

in Compare mode. The only exception to this is

when the CCP module is congured in Special

Event Trigger mode. In this mode, when Timer1

and CCPRx are equal, the CCPx interrupt

is generated, Timer1 is cleared, and an A/D

conversion is started (if the A/D module is

enabled.)

“Why Would I Use Compare Mode?”

Compare mode works much like the timer

function on a stopwatch. In the case of a

stopwatch, a predetermined time is loaded into

the watch and it counts down from that time

until zero is reached.

Compare works in the same way with

one exception – it counts from zero to the

predetermined time. This mode is useful for

generating specic actions at precise intervals.

A timer could be used to perform the same

functionality, however, it would mean preloading

the timer each time. Compare mode also has

the added benet of automatically altering the

state of the CCPx pin based on the way the

module is set up.

PIC

Microcontroller CCP and ECCP Tips ‘n Tricks

TIP #8 Modulation Formats

The CCP module, congured in Compare mode,

can be used to generate a variety of modulation

formats. The following gures show four

commonly used modulation formats:

Figure 8-1: Pulse-width Modulation

Figure 8-2: Manchester

TETE

TBP

Logic

‘ ’0

Logic

‘ ’1

Figure 8-3: Pulse Position Modulation

TETETE

TBP

Logic

‘ ’0

Logic

‘ ’1

T T TE E E

TBP

Logic

‘ ’0

Logic

‘ ’1

Figure 8-4: Variable Pulse-width Modulation

TBP

T TE E

TBP

Logic

‘ ’0

Logic

‘ ’1

Transition Low

to High

Transition High

to Low

TBP

T TE E

TBP

The gures show what a logic ‘0’ or a logic

‘1’ looks like for each modulation format.

A transmission typically resembles an

asynchronous serial transmission consisting of

a Start bit, followed by 8 data bits, and a Stop

bit.

Te is the basic timing element in each

modulation format and will vary based on the

desired baud rate.

Trigger Special Event mode can be used to

generate Te, (the basic timing element). When

the CCPx interrupt is generated, code in the ISR

routine would implement the desired modulation

format (additional modulation formats are also

possible).

PIC

Microcontroller CCP and ECCP Tips ‘n Tricks

TIP #9 Generating the Time Tick for a

RTOS

Real Time Operating Systems (RTOS) require

a periodic interrupt to operate. This periodic

interrupt, or “tick rate”, is the basis for the

scheduling system that RTOS’s employ. For

instance, if a 2 ms tick is used, the RTOS will

schedule its tasks to be executed at multiples

of the 2 ms. A RTOS also assigns a priority to

each task, ensuring that the most critical tasks

are executed rst. Table 9-1 shows an example

list of tasks, the priority of each task and the

time interval that the tasks need to be executed.

Table 9-1: Tasks

Task PriorityInterval

Read ADC Input 1 20 ms 2

Read ADC Input 2 60 ms 1

Update LCD 24 ms 2

Update LED Array 36 ms 3

Read Switch 10 ms 1

Dump Data to Serial Port 240 ms 1

The techniques described in Tip #7 can be used

to generate the 2 ms periodic interrupt using the

CCP module congured in Compare mode.

Note: For more information on RTOSs

and their use, see Application Note

AN777 “Multitasking on the

PIC16F877 with the Salvo™ RTOS”.

TIP #10 16-Bit Resolution PWM

Figure 10-1: 16-Bit Resolution PWM

CCPx Interrupt:

Clear CCPx pin

Timer1 Interrupt:

Set CCPx pin

1. Congure CCPx to clear output (CCPx pin)

on match in Compare mode (CCPxCON

<CCPSM3:CCPxM0>).

2. Enable the Timer1 interrupt.

3. Set the period of the waveform via Timer1

prescaler (T1CON <5:4>).

4. Set the duty cycle of the waveform using

CCPRxL and CCPRxH.

5. Set CCPx pin when servicing the Timer1

overow interrupt(1).

Note 1: One hundred percent duty cycle

is not achievable with this

implementation due to the interrupt

latency in servicing Timer1. The

period is not affected because

the interrupt latency will be the

same from period to period as long

as the Timer1 interrupt is serviced

rst in the ISR.

Timer1 has four congurable prescaler values.

These are 1:1, 1:2, 1:4 and 1:8. The frequency

possibilities of the PWM described above are

determined by Equation 10-1.

Equation 10-1

For a microcontroller running on a 20 MHz

oscillator (Fosc) this equates to frequencies

of 76.3 Hz, 38.1 Hz, 19.1 Hz and 9.5 Hz for

increasing prescaler values.

F = F /(65536*4*prescaler)pwm osc

PIC

Microcontroller CCP and ECCP Tips ‘n Tricks

TIP #11 Sequential ADC Reader

Figure 11-1: Timeline

Time

Read AN0 Read AN1 Read AN2 Read AN0

Trigger Special Event mode (a sub-mode in

Compare mode) generates a periodic interrupt

in addition to automatically starting an A/D

conversion when Timer1 matches CCPRxL and

CCPRxH. The following example problem

demonstrates how to sequentially read the A/D

channels at a periodic interval.

Example

Given the PIC16F684 running on its 8 MHz

internal oscillator, congure the microcontroller

to sequentially read analog pins AN0, AN1 and

AN2 at 30 ms intervals.

Step #1: Determine Timer1 Prescaler

a) Timer1 overows at: Tosc*4*65536*

prescaler.

b) For a prescaler of 1:1, the Timer1 overow

occurs in 32.8 ms.

c) This is greater than 30 ms, so a prescaler of

1 is adequate.

Step #2: Calculate CCPR1 (CCPR1L and

CCPR1H)

a) CCPR1 = Interval Time/(Tosc*4*prescaler) =

0.030/(125 ns*4*1) = 6000 = 0xEA60

b) Therefore, CCPR1L = 0x60, and CCPR1H =

0xEA

Step #3: Conguring CCP1CON

The ECCP module should be congured

in Trigger Special Event mode. This mode

generates an interrupt when Timer1 equals

the value specied in CCPR1. Timer1 is

automatically cleared and the GO bit in

ADCON0 is automatically set. For this mode,

CCP1CON = ‘ ’.b00001011

Step #4: Add Interrupt Service Routine Logic

When the ECCP interrupt is generated, select

the next A/D pin for reading by altering the

ADCON0 register.

PIC

Microcontroller CCP and ECCP Tips ‘n Tricks

TIP #12 Repetitive Phase Shifted

Sampling

Repetitive phase shifted sampling is a technique

to articially increase the sampling rate of an

A/D converter when sampling waveforms that

are both periodic and constant from period

to period. The technique works by capturing

regularly spaced samples of the waveform

from the start to nish of the waveform’s

period. Sampling of the next waveform is then

performed in the same manner, except that

the start of the sample sequence is delayed a

percentage of the sampling period. Subsequent

waveforms are also sampled, with each sample

sequence slightly delayed from the last, until

the delayed start of the sample sequence is

equal to one sample period. Interleaving the

sample sets then produces a sample set of

the waveform at a higher sample rate. Figure

12-1 shows an example of a high frequency

waveform.

Figure 12-1: High Frequency Periodic

Waveform

As indicated in the key, the nely dotted lines

show where the A/D readings are taken during

the rst period of the waveform. The medium

sized dashed lines show when the A/D readings

are taken during the second period, and so on.

Figure 12-2 shows these readings transposed

onto one period.

Figure 12-2: Transposed Waveform

Time

First Pass

Second Pass

Third Pass

Fourth Pass

Key

Volts

The CCP module is congured in Compare

Special Event Trigger mode to accomplish this

task. The phase shift is implemented by picking

values of CCPRxL and CCPRxH that are not

synchronous with the period of the sampling

waveform. For instance, if the period of a

waveform is 100 s, then sampling at a rate of m

once every 22 µs will give the following set of

sample times over 11 periods (all values in µs).

1st 2nd 3rd 4th 5th 6th 7th 8th 9th 10th 11th

0 10 20 8 18 6 16 4 14 2 12

22 32 42 30 40 28 38 26 36 24 34

44 54 64 52 62 50 60 48 58 46 56

66 76 86 74 84 72 82 70 80 68 78

88 98 96 94 92 90

When these numbers are placed in sequential

order, they reveal a virtual sampling interval (I ) v

of 2 s from 0 s to 100 s, although the actual m m m

sampling interval (I ) is 22 s.am

PIC

Microcontroller CCP and ECCP Tips ‘n Tricks

PWM TIPS ‘N TRICKS

The ECCP and CCP modules produce a 10-bit

resolution Pulse-Width Modulated (PWM)

waveform on the CCPx pin. The ECCP module

is capable of transmitting a PWM signal on one

of four pins, designated P1A through P1D. The

PWM modes available on the ECCP module

are:

• Single output (P1A only)

• Half-bridge output (P1A and P1B only)

• Full-bridge output forward

• Full-bridge output reverse

One of the following congurations must be

chosen when using the ECCP module in PWM

Full-bridge mode:

• P1A, P1C active-high; P1B, P1D active-high

• P1A, P1C active-high; P1B, P1D active-low

• P1A, P1C active-low; P1B, P1D active-high

• P1A, P1C active-low; P1B, P1D active-low

“Why Would I Use PWM Mode?”

As the next set of Tips ‘n Tricks demonstrate,

Pulse-Width Modulation (PWM) can be used

to accomplish a variety of tasks from dimming

LEDs to controlling the speed of a brushed DC

electric motor. All these applications are based

on one basic principle of PWM signals –

as the duty cycle of a PWM signal increases,

the average voltage and power provided by

the PWM increases. Not only does it increase

with duty cycle, but it increases linearly. The

following gure illustrates this point more clearly.

Notice that the RMS and maximum voltage are

functions of the duty cycle (DC) in the following

Figure 12-3.

Figure 12-3: Duty Cycle Relation to Vrms

Equation 12-1 shows the relation between Vrms

and V .max

Equation 12-1: Relation Between Vrms max and V

VMAX

VRMS

DC = 10%

VRMS

DC = 80%

Vrms max = DCxV

PIC

Microcontroller CCP and ECCP Tips ‘n Tricks

TIP #13 Deciding on PWM Frequency

In general, PWM frequency is application

dependent although two general rules-of-thumb

hold regarding frequency in all applications.

They are:

1. As frequency increases, so does current

requirement due to switching losses.

2. Capacitance and inductance of the load tend

to limit the frequency response of a circuit.

In low-power applications, it is a good idea

to use the minimum frequency possible to

accomplish a task in order to limit switching

losses. In circuits where capacitance and/or

inductance are a factor, the PWM frequency

should be chosen based on an analysis of the

circuit.

Motor Control

PWM is used extensively in motor control

due to the efciency of switched drive

systems as opposed to linear drives. An

important consideration when choosing PWM

frequency for a motor control application is

the responsiveness of the motor to changes

in PWM duty cycle. A motor will have a faster

response to changes in duty cycle at higher

frequencies. Another important consideration

is the sound generated by the motor. Brushed

DC motors will make an annoying whine

when driven at frequencies within the audible

frequency range (20 Hz-4 kHz.) In order to

eliminate this whine, drive brushed DC motors

at frequencies greater than 4 kHz. (Humans

can hear frequencies at upwards of 20 kHz,

however, the mechanics of the motor winding

will typically attenuate motor whine above

4 kHz).

LED and Light Bulbs

PWM is also used in LED and light dimmer

applications. Flicker may be noticeable with

rates below 50 Hz. Therefore, it is generally a

good rule to pulse-width modulate LEDs and

light bulbs at 100 Hz or higher.

TIP #14 Unidirectional Brushed DC

Motor Control Using CCP

Figure 14-1: Brushed DC (BDC) Motor

Control Circuit

PIC16F628

10 kΩ

22 pF

VCC

EMI/RFI

Suppression

Place on

motor

22 pF

Motor

100Ω

CCP1

Figure 14-1 shows a unidirectional speed

controller circuit for a brushed DC motor. Motor

speed is proportional to the duty cycle of the

PWM output on the CCP1 pin. The following

steps show how to congure the PIC16F628 to

generate a 20 kHz PWM with 50% duty cycle.

The microcontroller is running on a 20 MHz

crystal.

Step #1: Choose Timer2 Prescaler

a) F = Fosc/((PR2+1)*4*prescaler) = pwm

19531 Hz for PR2 = 255 and prescaler of 1

b) This frequency is lower than 20 kHz,

therefore a prescaler of 1 is adequate.

Step #2: Calculate PR2

PR2 = Fosc/(F *4*prescaler) – 1 = 249pwm

Step #3: Determine CCPR1L and

CCP1CON<5:4>

a) CCPR1L:CCP1CON<5:4> =

DutyCycle*0x3FF = 0x1FF

b) CCPR1L = 0x1FF >> 2 = 0x7F,

CCP1CON<5:4> = 3

Step #4: Congure CCP1CON

The CCP module is congured in PWM mode

with the Least Signicant bits of the duty cycle

set, therefore, CCP1CON = ‘ ’.b001111000

PIC

Microcontroller CCP and ECCP Tips ‘n Tricks

TIP #15 Bidirectional Brushed DC

Motor Control Using ECCP

Figure 15-1: Full-Bridge BDC Drive Circuit

10 kΩ

TC428

VCC

P1A

10 pF

Motor

10 pF

10 kΩ

P1B

TC428

10 kΩ

TC428

VCC

P1C

10 pF

10 kΩ

P1D

TC428

The ECCP module has brushed DC motor

control options built into it. Figure 15-1 shows

how a full-bridge drive circuit is connected

to a BDC motor. The connections P1A, P1B,

P1C and P1D are all ECCP outputs when the

module in congured in “Full-bridge Output

Forward” or “Full-bridge Output Reverse”

modes (CCP1CON<7:6>). For the circuit shown

in Figure 15-1, the ECCP module should be

congured in PWM mode: P1A, P1C active

high; P1B, P1D active high (CCP1CON<3:1>).

The reason for this is the MOSFET drivers

(TC428) are congured so a high input will turn

on the respective MOSFET.

The following table shows the relation between

the states of operation, the states of the ECCP

pins and the ECCP Conguration register.

State P1A P1B P1C P1D CCP1CON

Forward 1 tri-state tri-state mod ‘ ’b01xx1100

Reverse tri-state mod 1 tri-state ‘ ’b11xx1100

Coast tri-state tri-state tri-state tri-state N/A

Brake tri-state 1 1 tri-state N/A

Legend:

‘ ’ = high, ‘ ’ = low, mod = modulated, tri-state = 1 0

pin congured as input

PIC

Microcontroller CCP and ECCP Tips ‘n Tricks

R and C are chosen based on the following

equation:

Equation 16-2

Pick a value of C arbitrarily and then calculate

R. The attenuation of the PWM frequency for a

given RC lter is:

Equation 16-3

If the attenuation calculated in Equation 16-3

is not sufcient, then K must be increased in

Equation 16-1. See Application Note AN538

“Using PWM to Generate Analog Output in

PIC17C42” for more details on using PWM to

generate an analog output.

TIP #16 Generating an Analog Output

Figure 16-1: Low-Pass Filter

Op Amp

PIC16F684

CCP1 Analog

Out

Pulse-width modulated signals can be used to

create Digital-to-Analog (D/A) converters with

only a few external components. Conversion of

PWM waveforms to analog signals involves the

use of an analog low-pass lter. In order to

eliminate unwanted harmonics caused by a

PWM signal to the greatest degree possible, the

frequency of the PWM signal (F ) should be pwm

signicantly higher than the bandwidth (Fbw) of

the desired analog signal. Equation 16-1 shows

this relation.

Equation 16-1

Fpwm = K*Fbw

Where harmonics decrease as K increases

RC = 1/(2 F )pbw

Att(dB = – 10*log[1+(2 F RC)2]ppwm

PIC

Microcontroller CCP and ECCP Tips ‘n Tricks

TIP #18 Varying LED Intensity

The intensity of an LED can be varied by

pulse-width modulating the voltage across

the LED. A microcontroller typically drives an

LED with the circuit shown in Figure 18-1. The

purpose of R1 is to limit the LED current so

that the LED runs in its specied current and

voltage range, typically around 1.4 volts at

20 mA. Modulating the LED drive pin on the

microcontroller will vary the average current

seen by the LED and thus its intensity. As

mentioned in Tip #13, LEDs and other light

sources should be modulated at no less than

100 Hz in order to prevent noticeable icker.

Figure 18-1: LED Drive

PIC16F684

CCP1

270Ω

The CCP module, congured in PWM mode,

is ideal for varying the intensity of an LED.

Adjustments to the intensity of the LED are

made by simply varying the duty cycle of

the PWM signal driving the LED. This is

accomplished by varying the CCPRxL register

between 0 and 0xFF.

TIP #19 Generating X-10 ® Carrier

Frequency

X-10 uses a piggybacked 120 kHz square wave

(at 50% duty cycle) to transmit information over

60 Hz power lines. The CCP module, running

in PWM mode, can accurately create the 120

kHz square wave, referred to as the carrier

frequency. Figure 19-1 shows how the 120

kHz carrier frequency is piggybacked onto the

sinusoidal 60 Hz power waveform.

Figure 19-1: Carrier Frequency With

Sinusoidal Waveform

PIC16F87XA

0.1 Fµ

X2 Rated

200Ω

OSC2

OSC1

RC3/CCP1

120 CVA

7.680 MHz

High-Pass Filter

1 MΩ

50Ω

+5 VDC

X-10 species the carrier frequency at 120 kHz

(± 2 kHz). The system oscillator in Figure 18-1

is chosen to be 7.680 MHz, so that the CCP

module can generate precisely 120 kHz. X-10

requires that the carrier frequency be turned on

and off at different points on the 60 Hz power

waveform. This is accomplished by conguring

the TRIS register for the CCP1 pin as either

an input (carrier frequency off) or an output

(carrier frequency on). Refer to Application

Note AN236 “X-10 Home Automation Using the

PIC16F877A” for more details on X-10 and

for source code for setting up the CCP module

appropriately.

PIC

Microcontroller CCP and ECCP Tips ‘n Tricks

COMBINATION CAPTURE AND

COMPARE TIPS

The CCP and ECCP modules can be

congured on the y. Therefore, these modules

can perform different functions in the same

application provided these functions operate

exclusively (not at the same time). This section

will provide examples of using a CCP module in

different modes in the same application.

TIP #20 RS-232 Auto-baud

RS-232 serial communication has a variety

of baud rates to choose from. Multiple

transmission rates require software which

detects the transmission rate and adjusts the

receive and transmit routines accordingly.

Auto-baud is used in applications where

multiple transmission rates can occur. The CCP

module can be congured in Capture mode to

detect the baud rate and then be congured in

Compare mode to generate or receive RS-232

transmissions.

In order for auto-baud to work, a known

calibration character must be transmitted initially

from one device to another. One possible

calibration character is show in Figure 20-1.

Timing this known character provides the

device with the baud rate for all subsequent

communications.

Figure 20-1: RS-232 Calibration Character

Start

Bit Stop

Bit

LSB MSB

0 0 0 0 0 0 0 1

Auto-baud Routine Implementation:

1. Congure CCP module to capture the falling

edge (beginning of Start bit).

2. When the falling edge is detected, store the

CCPR1 value.

3. Congure the CCP module to capture the

rising edge.

4. Once the rising edge is detected, store the

CCPR1 value.

5. Subtract the value stored in step 2 from the

value in step 4. This is the time for 8 bits.

6. Shift the value calculated in step 5 right 3

times to divide by 8. This result is the period

of a bit (T ).b

7. Shift value calculated in step 6 right by 1.

This result is half the period of a bit.

The following code segments show the process

for transmitting and receiving data in the normal

program ow. This same functionality can

be accomplished using the CCP module by

conguring the module in Compare mode and

generating a CCP interrupt every bit period.

When this method is used, one bit is either sent

or received when the CCP interrupt occurs.

Note: Refer to Application Note AN712

“RS-232 Auto-baud for the PIC16C5X

Devices” for more details on

auto-baud.

PIC

Microcontroller CCP and ECCP Tips ‘n Tricks

Example 20-1: Transmit Routine

TxRoun neti

MO reload bit co teVL ;pW 8 un r

;with 8

MOVWF counter

BCF TxLine ;line initially high,

;toggle lo for STARw T

;bit

TxLoop

CA layTLL De b ;wait Tb (bit period)

RRF RxByte,f ;rota LS first inte B to

;the Carry flag

BTFSS STATUS,C ;Tx line state ualseq

;state of rry flagCa

BCF TxLine

BTFSC STATUS,C

BSF TxLine

DE SZ unter,CF Co f ;Repe 8 meat ti s

GO LoopTO Tx

CA lay Tb elay Tb foreLL De ;D be

;sending STOP bit

BSF TxLine ;send STOP bit

Example 20-2: Receive Routine

RxRoutine

BTFSC RxLi ;wait for receivene

;line to go low

GOTO RxRo inut e

MOVLW 8 ;initialize bit

;counter to 8

MOVWF Counter

CALL Dela alfTb;delay 1/2 Tb herey1H

;plus Tb in RxLoop

;in order to sample

;at the ght timeri

RxLoop

CALL DelayTb ;wa Tb (biit t

;period)

BTFSS RxLi ;Carry flag statene

;equals liRx ne

;state

BC USF STAT ,C

BTFSC RxLine

BS USF STAT ,C

BTFSC RxLine

BS USF STAT ,C

RR teF RxBy ,f ;Rotate LSB first

;into receive type

DECFSZ Coun r,te f ;Repeat time8 s

GOTO RxLoop

PIC

Microcontroller CCP and ECCP Tips ‘n Tricks

TIP #21 Dual-Slope Analog-to-Digital

Converter

A circuit for performing dual-slope A/D

conversion utilizing the CCP module is shown in

Figure 21-1.

Figure 21-1: Dual-Slope Analog-to-Digital

Converter

Comparator

(on-board )PIC16F684

Integrator

-V

REF

CCP1

PIC16F684

Dual-slope A/D conversion works by integrating

the input signal (Vin) for a xed time (T1). The

input is then switched to a negative reference

(-Vref) and integrated until the integrator output

is zero (T2). V is a function of Vin ref and the

ratio of T2 to T1.

Figure 21-2: V vs. Time

Time

VIN

Integrator output

with VIN input

VIN

Integrator output

with VREF input

T1 T2

The components of this conversion type are

the xed time and the timing of the falling edge.

The CCP module can accomplish both of these

components via Compare mode and Capture

mode respectively. Here’s how:

1. Congure the CCP module in Compare

mode, Special Event Trigger.

2. Switch the analog input into the integrator

from Vref to Vin.

3. Use the CCP module to wait T1 (T1 chosen

based on capacitor value).

4. When the CCP interrupt occurs, switch the

analog input into the regulator from V to in

Vref and recongure the module in Capture

mode; wait for falling edge.

5. When the next CCP interrupt occurs, the time

captured by the module is T2.

6. Calculate V using Equation 21-1.in

Equation 21-1

V = Vin ref T2

PIC

Microcontroller CCP and ECCP Tips ‘n Tricks

NOTES:

PIC

Microcontroller Comparator Tips ‘n Tricks

Table Of Contents

TIPS ‘N TRICKS INTRODUCTION

TIP #1: Low Battery Detection ........................ 4-2

TIP #2: Faster Code for Detecting Change ..... 4-3

TIP #3: Hysteresis ........................................... 4-4

TIP #4: Pulse Width Measurement ................. 4-5

TIP #5: Window Comparison .......................... 4-6

TIP #6: Data Slicer .......................................... 4-7

TIP #7: One-Shot ............................................ 4-8

TIP #8: Multi-Vibrator (Square Wave Output) 4-9 .

TIP #9: Multi-Vibrator (Ramp Wave Output) ... 4-10

TIP #10: Capacitive Voltage Doubler ................ 4-11

TIP #11: PWM Generator .................................4-12

TIP #12: Making an Op Amp Out of a

Comparator ........................................4-13

TIP #13: PWM High-Current Driver ..................4-14

TIP #14: Delta-Sigma ADC ............................... 4-15

TIP #15: Level Shifter .......................................4-16

TIP #16: Logic: Inverter ..................................... 4-16

TIP #17: Logic: AND/NAND Gate .....................4-17

TIP #18: Logic: OR/NOR Gate .......................... 4-18

TIP #19: Logic: XOR/XNOR Gate .....................4-19

TIP #20: Logic: Set/Reset Flip Flop .................. 4-20

TIPS ‘N TRICKS INTRODUCTION

Microchip continues to provide innovative

products that are smaller, faster, easier to

use and more reliable. The Flash-based

PIC® microcontrollers (MCUs) are used in a

wide range of everyday products from smoke

detectors to industrial, automotive and medical

products.

The PIC12F/16F Family of devices with on-chip

voltage comparators merge all the advantages

of the PIC MCU architecture and the exibility

of Flash program memory with the mixed signal

nature of a voltage comparator. Together they

form a low-cost hybrid digital/analog building

block with the power and exibility to work in an

analog world.

The exibility of Flash and an excellent

development tool suite, including a low-

cost In-Circuit Debugger, In-Circuit Serial

Programming™ (ICSP™) and MPLAB ® ICE

2000 emulation, make these devices ideal for

just about any embedded control application.

The following series of Tips ‘n Tricks can be

applied to a variety of applications to help

make the most of discrete voltage comparators

or microcontrollers with on-chip voltage

comparators.

CHAPTER 4

PIC ® Microcontroller Comparator

Tips ‘n Tricks

PIC

Microcontroller Comparator Tips ‘n Tricks

Figure 1-2: Unregulated Supply

Low Battery

Enable

BATT

Comparator

Comparator will trip when V = 3V: R1 = 33k, batt

R2 = 10k and R3 = 470Ω.

In Figure 1-2, resistor R3 is chosen to bias

diode D1 above its forward voltage when V batt

is equal to the minimum battery voltage for the

system. Resistors R1 and R2 are chosen to set

the inverting input voltage equal to the forward

voltage of D1.

TIP #1 Low Battery Detection

When operating from a battery power supply, it

is important for a circuit to be able to determine

when the battery charge is insufcient for

normal operation of the circuit. Typically, this

is a comparator-based circuit similar to the

Programmable Low Voltage Detect (PLVD)

peripheral. If the PLVD peripheral is not

available in the microcontroller, a similar circuit

can be constructed using a comparator and a

few external components (see Figure 1-1 and

Figure 1-2). The circuit in Figure 1-1 assumes

that the microcontroller is operating from a

regulated supply voltage. The circuit in Figure

1-2 assumes that the microcontroller supply is

unregulated.

Figure 1-1: Regulated Supply

Low Battery

Enable

BATT

The comparator will trip when the battery

voltage, V = 5.7V: R1 = 33k, R2 = 10k, batt

R3 = 39k, R4 = 10k, V = 5V.dd

In Figure 1-1, resistors R1 and R2 are chosen

to place the voltage at the non-inverting input

at approximately 25% of Vdd. R3 and R4

are chosen to set the inverting input voltage

equal to the non-inverting input voltage when

the battery voltage is equal to the minimum

operating voltage for the system.

PIC

Microcontroller Comparator Tips ‘n Tricks

TIP #2 Faster Code for Detecting

Change

When using a comparator to monitor a sensor,

it is often just as important to know when

a change occurs as it is to know what the

change is. To detect a change in the output of

a comparator, the traditional method has been

to store a copy of the output and periodically

compare the held value to the actual output to

determine the change. An example of this type

of routine is shown below.

Example 2-1

Test

MOVF hold,w ;get old Cout

XORWF CMCON,w ;compare to new Cout

ANDLW COUTMASK

BTFSC STATUS,Z

RETLW 0 ;if = return "no change"

MOVF CMCON,w ;if not =, get new Cout

ANDLW COUTMASK ;remove all other bits

MOVWF hold ;store in holding var.

IORLW CHNGBIT ;add change flag

RETURN

This routine requires 5 instructions for each test,

9 instructions if a change occurs, and 1 RAM

location for storage of the old output state.

A faster method for microcontrollers with a

single comparator is to use the comparator

interrupt ag to determine when a change has

occurred.

Example 2-2

Test

BTFSS PIR1,CMIF ;test comparator flag

RETLW 0 ;if clear, return a 0

BTFSS CMCON,COUT ;test Cout

RETLW CHNGBIT ;if clear return

;CHNGFLAG

RETLW COUTMASK + CHNGBIT;if set,

;return both

This routine requires 2 instructions for each test,

3 instructions if a change occurs, and no RAM

storage.

If the interrupt ag can not be used, or if two

comparators share an interrupt ag, an alternate

method that uses the comparator output polarity

bit can be used.

Example 2-3

Test

BTFSS CMCON,COUT ;test Cout

RETLW 0 ;if clear, return 0

MOVLW CINVBIT ;if set, invert Cout

XORWF CMCON,f ;forces Cout to 0

BTFSS CMCON,CINV ;test Cout polarity

RETLW CHNGFLAG ;if clear, return

;CHNGFLAG

RETLW COUTMASK + CHNGFLAG;if set,

;return both

This routine requires 2 instructions for each test,

5 instructions if a change occurs, and no GPR

storage.

PIC

Microcontroller Comparator Tips ‘n Tricks

TIP #3 Hysteresis

When the voltages on a comparator’s input are

nearly equal, external noise and switching noise

from inside the microcontroller can cause the

comparator output to oscillate or “chatter.” To

prevent chatter, some of the comparator output

voltage is fed back to the non-inverting input of

the comparator to form hysteresis (see Figure

3-1). Hysteresis moves the comparator

threshold up when the input is below the

threshold, and down when the input is above

the threshold. The result is that the input must

overshoot the threshold to cause a change

in the comparator output. If the overshoot is

greater than the noise present on the input, the

comparator output will not chatter.

Figure 3-1: Comparator with Hysteresis

Input

R3R1

Output

To calculate the resistor values required, rst

determine the high and low threshold values

which will prevent chatter (V and V ). Using th tl

V and V , the average threshold voltage can th tl

be calculated using the equation.

Equation 3-1

Next, choose resistor values that satisfy

Equation 3-2 and calculate the equivalent

resistance using Equation 3-3.

Note: A continuous current will ow through

R1 and R2. To limit the power

dissipation in R1 and R2 the total

resistance of R1 and R2 should be at

least 1k. The total resistance of R1

and R2 should also be kept below

10K to keep the size of R3 small.

Large values for R3, 100k-10 M

can produce voltage offsets at the

non-inverting input due to the

comparator’s input bias current.

Equation 3-2

Equation 3-3

Then, determine the feedback divider ratio D , r

using Equation 3-4.

Equation 3-4

Finally, calculate the feedback resistor R3 using

Equation 3-5.

Equation 3-5

Example:

• A Vdd = 5.0V, V = 3.0V and V = 2.5Vh l

• Vavg = 2.77V

• R = 8.2k and R2 = 10k, gives a Vavg = 2.75V

• Req = 4.5k

• Dr = .1

• R3 = 39k (40.5 calculated)

• Vhact = 2.98V

• Vlact = 2.46V

R3 = Req [ ( 1 ) - 1]

Vavg = V R2 dd *

R1 + R2

Req = R1 R2 *

R1 + R2

Dr = (V V th - tl)

Vdd

Vavg = V V dd * tl

Vdd - + Vth Vtl

PIC

Microcontroller Comparator Tips ‘n Tricks

TIP #4 Pulse Width Measurement

To measure the high or low pulse width of an

incoming analog signal, the comparator can

be combined with Timer1 and the Timer1 Gate

input option (see Figure 4-1). Timer1 Gate acts

as a count enable for Timer1. If the input is

low, Timer1 will count. If the T1G input is high,

Timer1 does not count. Combining T1G with the

comparator allows the designer to measure the

time between a high-to-low output change and a

low-to-high output change.

To make a measurement between a low-to-high

and a high-to-low transition, the only change

required is to set the CINV bit in the comparator

CMCON register which inverts the comparator

output.

Because the output of the comparator can

change asynchronously with the Timer1 clock,

only comparators with the ability to synchronize

their output with the Timer1 clock should be

used and their C2SYNC bits should be set.

Figure 4-1: Comparator with Timer1 and T1G

Trigger

Level

INPUT

OUT

Timer1

TIG

If the on-chip comparator does not have the

ability to synchronize its output to the Timer1

clock, the output can be synchronized externally

using a discrete D ip-op (see Figure 4-2).

Note: The ip-op must be falling edge

triggered to prevent a race condition.

Figure 4-2: Externally Synchronized

Comparator

Trigger

Level

VDD

VINPUT

COUT

Timer1

TIG

Timer

Clock T1CKI

D O

PIC

Microcontroller Comparator Tips ‘n Tricks

TIP #5 Window Comparison

When monitoring an external sensor, it is often

convenient to be able to determine when the

signal has moved outside a pre-established

safe operating range of values or window of

operation. This windowing provides the circuit

with an alarm when the signal moves above or

below safety limits, ignoring minor uctuations

inside the safe operating range.

To implement a window comparator, two voltage

comparators and 3 resistors are required (see

Figure 5-1).

Figure 5-1: Window Comparator

Resistors R1, R2 and R3 form a voltage divider

which generates the high and low threshold

voltages. The outputs HIGH LIMIT and LOW

LIMIT are both active high, generating a logic

one on the HIGH LIMIT output when the input

voltage rises above the high threshold, and a

logic one on the LOW LIMIT output when the

input voltage falls below the low threshold.

To calculate values for R1, R2 and R3, nd

values that satisfy Equation 5-1 and Equation

5-2.

Note:

A continuous current will ow

through R1, R2 and R3. To limit the

power dissipation in the resistors, the

total resistance of R1, R2 and R3

should be at least 1k. The total

resistance of R1, R2 and R3 should

also be kept less than 1

W to

prevent offset voltages due to the

input bias currents of the comparator.

Equation 5-1

Equation 5-2

Example:

• Vdd = 5.0V, V = 2.5V, V = 2.0Vth tl

• R1 = 12k, R2 = 2.7k, R3 = 10k

• Vth (actual) = 2.57V, V (actual) = 2.02Vtl

Adding Hysteresis:

To add hysteresis to the HIGH LIMIT

comparator, follow the procedure outlined in

Tip #3. Use the series combination of R2 and

R3 as the resistor R2 in Tip #3.

To add hysteresis to the LOW LIMIT

comparator, choose a suitable value for Req,

1k to 10 k

, and place it between the circuit

input and the non-inverting input of the LOW

LIMIT comparator. Then calculate the needed

feedback resistor using Equation 3-4 and

Equation 3-5.

Input

Low Limit

High Limit

Vth- = hi V R +R ) dd * ( 3 2

R1 + R2 + R3

Vth- = lo V R dd * 3

R1 + R2 + R3

PIC

Microcontroller Comparator Tips ‘n Tricks

TIP #6 Data Slicer

In both wired and wireless data transmission,

the data signal may be subject to DC offset

shifts due to temperature shifts, ground

currents or other factors in the system. When

this happens, using a simple level comparison

to recover the data is not possible because

the DC offset may exceed the peak-to-peak

amplitude of the signal. The circuit typically

used to recover the signal in this situation is a

data slicer.

The data slicer shown in Figure 6-1 operates

by comparing the incoming signal with a sliding

reference derived from the average DC value

of the incoming signal. The DC average value

is found using a simple RC low-pass lter (R1

and C1). The corner frequency of the RC lter

should be high enough to ignore the shifts in

the DC level while low enough to pass the data

being transferred.

Resistors R2 and R3 are optional. They provide

a slight bias to the reference, either high or low,

to give a preference to the state of the output

when no data is being received. R2 will bias the

output low and R3 will bias the output high. Only

one resistor should be used at a time, and its

value should be at least 50 to 100 times larger

than R1.

Figure 6-1: Data Slicer

Input

VDD

Output

Comparator

Example:

Data rate of 10 kbits/second. A low pass lter

frequency of 500 Hz: R1 = 10k, C1 = 33 F. R2 m

or R3 should be 500k to 1 MB.

PIC

Microcontroller Comparator Tips ‘n Tricks

When the voltage across C1 exceeds the high

threshold voltage, the output of the comparator

goes low, C1 is discharged to just above the

0.7V limit, the non-inverting input is pulled below

0.7V, and the circuit is reset for the next pulse

input, waiting for the next trigger input.

Figure 7-1: One-Shot Circuit

Input

VDD

VDD Output

Comparator

To design the one-shot, rst create the

hysteresis feedback using the techniques from

Tip #3. Remember to set the low threshold

below 0.7V. Next, choose values for R2 and C1

using Equation 7-1.

Equation 7-1

D1 can be any low voltage switching diode. R1

should be 1% to 2% of R2 and C2 should be

between 100 and 220 pF.

Example:

• Vdd = 5V, V = 3.0V, V = 2.5Vth tl

• From Tip #3, R4 = 1k, R5 = 1.5k and R3 = 12k

• Tpulse = I , C1 = .1 F and R2 = 15kms m

• D1 is a 1N4148, R1 = 220W and C2 = 150 pF

TIP #7 One-Shot

When dealing with short duration signals or

glitches, it is often convenient to stretch out the

event using a mono-stable, multi-vibrator or

one-shot. Whenever the input pulses, the

one-shot res holding its output for a preset

period of time. This stretches the short

trigger input into a long output which the

microcontroller can capture.

The circuit is designed with two feedback paths

around a comparator. The rst is a positive

hysteresis feedback which sets a two level

threshold, V and V , based on the state of hi lo

the comparator output. The second feedback

path is an RC time circuit.

The one-shot circuit presented in Figure 7-1 is

triggered by a low-high transition on its input

and generates a high output pulse. Using

the component values from the example, the

circuit’s operation is as follows.

Prior to triggering, C1 will have charged to

a voltage slightly above 0.7V due to resistor

R2 and D1 (R1 << R2 and will have only a

minimal effect on the voltage). The comparator

output will be low, holding the non-inverting

input slightly below 0.7V due to the hysteresis

feedback through R3, R4 and R5 (the hysteresis

lower limit is designed to be less than 0.7V).

With the non-inverting input held low, C2 will

charge up to the difference between the

circuit input and the voltage present at the

non-inverting input.

When the circuit input is pulsed high, the

voltage present at the non-inverting input is

pulled above 0.7V due to the charge in C2.

This causes the output of the comparator to go

high, the hysteresis voltage at the non-inverting

input goes to the high threshold voltage, and C1

begins charging through R2.

Tpulse = R2 * C1 * In(V ) th/vtl

PIC

Microcontroller Comparator Tips ‘n Tricks

To design a multi-vibrator, rst design the

hysteresis feedback path using the procedure in

Tip #3. Be careful to choose threshold voltages

(V and V ) that are evenly spaced within the th tl

common mode range of the comparator and

centered on V /2. Then use V and V to dd dd tl

calculate values for RT and CT that will result in

the desired oscillation frequency F . Equation osc

8-1 denes the relationship between RT, CT,

V , V and F .th tl osc

Equation 8-1

Example:

• Vdd = 5V, V = 3.333, V = 1.666Vth tl

• R1, to R2, to R3 = 10k

• Rt t = 15 kHz, C = .1 F for Fm osc = 480 Hz

TIP #8 Multi-Vibrator (Square Wave

Output)

A multi-vibrator is an oscillator designed around

a voltage comparator or operational amplier

(see Figure 8-1). Resistors R1 through R3

form a hysteresis feedback path from the

output to the non-inverting input. Resistor RT

and capacitor CT form a time delay network

between the output and the inverting input. At

the start of the cycle, CT is discharged holding

the non-inverting input at ground, forcing

the output high. A high output forces the

non-inverting input to the high threshold voltage

(see Tip #3) and charges CT through RT.

When the voltage across CT reaches the high

threshold voltage, the output is forced low. A low

output drops the non-inverting input to the low

threshold voltage and discharges CT through

RT. When the voltage across CT reaches the

low threshold voltage, the output is forced high

and the cycle starts over.

Figure 8-1: Multi-Vibrator Circuit

Output

Comparator

Fosc = 1

2 * RT * CT * In(V

)

PIC

Microcontroller Comparator Tips ‘n Tricks

TIP #9 Multi-Vibrator (Ramp Wave

Output)

A multi-vibrator (ramp wave output) is an

oscillator designed around a voltage comparator

or operational amplier that produces an

asymmetrical output waveform (see Figure 9-1).

Resistors R1 through R3 form a hysteresis

feedback path from the output to the

non-inverting input. Resistor RT, diode D1 and

capacitor CT form a time delay network between

the output and the inverting input. At the start

of the cycle, CT is discharged holding the

non-inverting input at ground, forcing the output

high. A high output forces the non-inverting

input to the high threshold voltage (see Tip #3)

and charges CT through RT. When the voltage

across CT reaches the high threshold voltage,

the output is forced low. A low output drops the

non-inverting input to the low threshold voltage

and discharges CT through D1. Because

the dynamic on resistance of the diode is

signicantly lower than RT, the discharge of CT

is small when compared to the charge time, and

the resulting waveform across CT is a pseudo

ramp function with a ramping charge phase and

a short-sharp discharge phase.

Figure 9-1: Ramp Waveform Multi-Vibrator

Comparator

Output

To design this multi-vibrator, rst design the

hysteresis feedback path using the procedure

in Tip #3. Remember that the peak-to-peak

amplitude of the ramp wave will be determined

by the hysteresis limits. Also, be careful to

choose threshold voltages (V and V ) that th tl

are evenly spaced within the common mode

range of the comparator.

Then use V and V to calculate values for RT th tl

and CT that will result in the desired oscillation

frequency Fosc. Equation 9-1 denes the

relationship between RT, CT, V , V and F .th tl osc

Equation 9-1

This assumes that the dynamic on resistance of

D1 is much less than RT.

Example:

• Vdd = 5V, V = 1.666V and V = 3.333Vth th

• R1, R2 and R3 = 10k

• Rt t = 15k, C = .1 F for a Fm osc = 906 Hz

Note:

Replacing R with a current limiting t

diode will signicantly improve the

linearity of the ramp wave form.

Using the example shown above, a

CCL1000 (1 mA Central Semiconductor

CLD), will produce a very linear

6 kHz output (see Equation 9-2).

Equation 9-2

Figure 9-2: Alternate Ramp Waveform

Multi-Vibrator Using a CLD

CLD

-Comparator

Output

VDD

Fosc = 1

RT * CT * In(V

)

Fosc = I cld

C (V

- V

)

PIC

Microcontroller Comparator Tips ‘n Tricks

Figure 10-2: Equivalent Output Model

VOUT

ROUT

2 x VDD

To design a voltage doubler, rst determine the

maximum tolerable output resistance based on

the required output current and the minimum

tolerable output voltage. Remember that the

output current will be limited to one half of

the output capability of the comparator. Then

choose a transfer capacitance and switching

frequency using Equation 10-1.

Equation 10-1

Note: Rout will be slightly higher due to

the dynamic resistance of the diodes.

The equivalent series resistance or

ESR, of the capacitors and the

output resistance of the comparator.

See the TC7660 data sheet for a

more complete description.

Once the switching frequency is determined,

design a square-wave multi-vibrator as

described in Tip #8.

Finally, select diodes D1 and D2 for their current

rating and set C2 equal to C1.

Example:

From Tip #8, the values are modied for a Fosc

of 4.8 kHz.

• C1 and C2 = 10 mF

• Rout = 21W

TIP #10 Capacitive Voltage Doubler

This tip takes the multi-vibrator described in

Tip #8 and builds a capacitive voltage doubler

around it (see Figure 10-1). The circuit works

by alternately charging capacitor C1 through

diode D1, and then charge balancing the energy

in C1 with C2 through diode D2. At the start

of the cycle, the output of the multi-vibrator is

low and charge current ows from Vdd through

D1 and into C1. When the output of the multi-

vibrator goes high, D1 is reverse biased and the

charge current stops. The voltage across C1 is

added to the output voltage of the multi-vibrator,

creating a voltage at the positive terminal of C1

which is 2 x Vdd. This voltage forward biases D2

and the charge in C1 is shared with C2. When

the output of the multi-vibrator goes low again,

the cycle starts over.

Figure 10-1: Capacitive Voltage Doubler

OLT

Comparator

R1 C2

Note: The output voltage of a capacitive

double is unregulated and will sag

with increasing load current.

Typically, the output is modeled as a

voltage source with a series

resistance (see Figure 10-2).

Rout = 1

swi t ch

* C1

PIC

Microcontroller Comparator Tips ‘n Tricks

TIP #11 PWM Generator

This tip shows how the multi-vibrator (ramp wave)

can be used to generate a voltage controlled

PWM signal. The ramp wave multi-vibrator

operates as described in Tip #9, generating a

positive going ramp wave. A second comparator

compares the instantaneous voltage of the ramp

wave with the incoming voltage to generate the

PWM output (see Figure 11-2).

When the ramp starts, it is below the input

voltage, and the output of the second

comparator is pulled high starting the PWM

pulse. The output remains high until the ramp

wave voltage exceeds the input, then the output

of the second comparator goes low ending

the PWM pulse. The output of the second

comparator remains low for the remainder of

the ramp waveform. When the ramp waveform

returns to zero at the start of the next cycle, the

second comparator output goes high again and

the cycle starts over.

Figure 11-1: PWM Wave Forms

Ramp

Wave

Form

Time

Output

Input

Time

Figure 11-2: PWM Circuit

CLD

Comparator

Input +

-Comparator

Output

To design a PWM generator, start with the

design of a ramp wave multi-vibrator using the

design procedure from Tip #9. Choose high and

low threshold voltages for the multi-vibrators

hysteresis feedback that are slightly above and

below the desired PWM control voltages.

Note: The PWM control voltage will

produce a 0% duty cycle for inputs

below the low threshold of the

multi-vibrator. A control voltage

greater than the high threshold

voltage will produce a 100% duty

cycle output.

Using the example values from Tip #9 will result

in a minimum pulse width at an input voltage of

1.7V and a maximum at an input of 3.2V.

PIC

Microcontroller Comparator Tips ‘n Tricks

Once the gain has been determined, values for

R3 and C2 can be determined. R3 and C2 form

a low-pass lter on the output of the amplier.

The corner frequency of the low pass should

be 2 to 3 times the maximum frequency of the

signal being amplied to prevent attenuation

of the signal, and R3 should be kept small to

minimize the output impedance of the amplier.

Equation 12-2 shows the relationship between

R3, C2 and the corner frequency of the low

pass lter.

Equation 12-2

A value for C1 can then be determined using

Equation 12-3. The corner frequency should be

the same as Equation 12-3.

Equation 12-3

To design an inverting amp, choose resistors R1

and R2 using the Gain formula for an op amp

inverting amplier (see Equation 12-4).

Equation 12-4

Then choose values for the resistor divider

formed by R4 and R5. Finally choose C1 and

C2 as shown in the non-inverting amplier

design.

Example:

• For C2 will set the corner F

• Gain = 6.156, R1 = R3 = 19.8k

• R2 = 3.84k, C1 = .047 mF, F = 171 Hzcorner

• C2 = .22 mF

TIP #12 Making an Op Amp

Out of a Comparator

When interfacing to a sensor, some gain is

typically required to match the full range of the

sensor to the full range of an ADC. Usually this

is done with an operational amplier, however,

in cost sensitive applications, an additional

active component may exceed the budget.

This tip shows how an on-chip comparator can

be used as an op amp like gain stage for slow

sensor signals. Both an inverting and

non-inverting topology are shown (see Figure

12-1 and Figure 12-2).

Figure 12-1: Non-Inverting Amplier

Output

Comparator

Input

Figure 12-2: Inverting Amplier

VDD

10K

Output

Comparator

Input

10K

To design a non-inverting amplier, choose

resistors R1 and R2 using the Gain formula for

an op amp non-inverting amplier (see Equation

12-1).

Equation 12-1

Gain = R1 + R2

Fcorner = 1

2 * * R3 * C2W

Fcorner = 1

2 * * (R1 II R2) * C2W

Gain = R 1

PIC

Microcontroller Comparator Tips ‘n Tricks

TIP #13 PWM High-Current Driver

This tip combines a comparator with a MOSFET

transistor and an inductor to create a switch mode

high-current driver circuit. (See Figure 13-1).

The operation of the circuit begins with the

MOSFET off and no current owing in the

inductor and load. With the sense voltage across

R1 equal to zero and a DC voltage present at

the drive level input, the output of the comparator

goes low. The low output turns on the MOSFET

and a ramping current builds through the

MOSFET, inductor, load and R1.

Figure 13-1: High Current Driver

Drive

Level

Comparator R3

P ch

MOSFET

Load

When the current ramps high enough to generate

a voltage across R1 equal to the drive level, the

comparator output goes high turning off the

MOSFET. The voltage at the junction of the

MOSFET and the inductor then drops until D1

forward biases. The current continues ramping

down from its peak level toward zero. When the

voltage across the sense resistor R1 drops below

the drive level, the comparator output goes low,

the MOSFET turns on, and the cycle starts over.

R2 and C1 form a time delay network that limits

the switching speed of the driver and causes it

to slightly overshoot and undershoot the drive

level when operating. The limit is necessary to

keep the switching speed low, so the MOSFET

switches efciently. If R2 and C1 were not

present, the system would run at a speed set

by the comparator propagation delay and the

switching speed of the MOSFET. At that speed,

the switching time of the MOSFET would be a

signicant portion of the switching time and the

switching efciency of the MOSFET would be too

low.

Figure 13-1: Current Through the Load

Time

Drive Level Ripple Current

Load Current

To design a PWM high current driver, rst

determine a switching speed (F ) that is swx

appropriate for the system. Next, choose a

MOSFET and D1 capable of handling the load

current requirements. Then choose values for

R2 and C1 using Equation 13-1.

Equation 13-1

Next determine the maximum ripple current that

the load will tolerate, and calculate the required

inductance value for L1 using Equation 13-2.

Equation 13-2

Finally, choose a value for R1 that will produce

a feedback ripple voltage of 100 mV for the

maximum ripple current Iripple.

Example:

• Fswx = 10 kHz, R2 = 22k, C1 = .01 Fm

• Iripple = 100 mA, V = 12V, V = 3.5Vdd l

• L = 4.25 mH

Fswx = 2

R2 * C1

L = V - V dd load

ripple

* F

swx

* 2

PIC

Microcontroller Comparator Tips ‘n Tricks

TIP #14 Delta-Sigma ADC

This tip describes the creation of a hardware/

software-based Delta-Sigma ADC. A Delta-

Sigma ADC is based on a Delta-Sigma

modulator composed of an integrator, a

comparator, a clock sampler and a 1-bit DAC

output. In this example, the integrator is formed

by R1 and C1. The comparator is an on-chip

voltage comparator. The clock sampler is

implemented in software and the 1-bit DAC

output is a single I/O pin. The DAC output feeds

back into the integrator through R2.

Resistors R3 and R4 form a V /2 reference for dd

the circuit (see Figure 14-1).

Figure 14-1: Delta-Sigma Modulator

-Comparator

R1 R2

Input

Software Data

In operation, the feedback output from the

software is a time sampled copy of the

comparator output. In normal operation, the

modulator output generates a PWM signal

which is inversely proportional to the input

voltage. As the input voltage increases,

the PWM signal will drop in duty cycle to

compensate. As the input decreases, the duty

cycle rises.

To perform an A-to-D conversion, the duty

cycle must be integrated over time, digitally, to

integrate the duty cycle to a binary value. The

software starts two counters. The rst counts

the total number of samples in the conversion

and the second counts the number of samples

that were low. The ratio of the two counts is

equal to the ratio of the input voltage over V . dd

Note: This assumes that R1 and R2 are

equal and R3 is equal to R4. If

R1 and R2 are not equal, then the

input voltage is also scaled by the

ratio of R2 over R1, and R3 must still

be equal to R4.

For a more complete description of the

operation of a Delta-Sigma ADC and example

rmware, see Application Note AN700 “Make A

Delta-Sigma Converter Using a Microcontroller’s

Analog Comparator Module.”

Example:

• R3 = R4 = 10 kHz

• R1 = R2 = 5.1k

• C1 = 1000 pF

PIC

Microcontroller Comparator Tips ‘n Tricks

TIP #15 Level Shifter

This tip shows the use of the comparator as a

digital logic level shifter. The inverting input is

biased to the center of the input voltage range

(V /2). The non-inverting input is then used for in

the circuit input. When the input is below the

V /2 threshold, the output is low. When the in

input is above V /2, then the output is high. in

Values for R1 and R2 are not critical, though

their ratio should result in a threshold voltage

V /2 at the mid-point of the input signal voltage in

range. Some microcontrollers have the option to

connect the inverting input to an internal voltage

reference. To use the reference in place of R1

and R2, simply select the internal reference and

congure it for one half the input voltage range.

Note: Typical propagation delay for the

circuit is 250-350 ns using the typical

on-chip comparator peripheral of a

microcontroller.

Figure 15-1: Level Shifter

VDD

Example:

• Vin = 0 - 2V, V /2 = 1V, V = 5Vin dd

• R2 = 10k, R3 = 3.9k

TIP #16 Logic: Inverter

When designing embedded control applications,

there is often the need for an external gate.

Using the comparator, several simple gates can

be implemented. This tip shows the use of the

comparator as an inverter.

The non-inverting input is biased to the center

of the input voltage range, typically V /2. The dd

inverting input is then used for the circuit input.

When the input is below V /2, the output is dd

high. When the input is above V /2, then the dd

output is low.

Values for R1 and R2 are not critical, though

they must be equal to set the threshold at

V /2. dd

Some microcontrollers have the option to

connect the inverting input to an internal voltage

reference. To use the reference in place of R1

and R2, move the input to the non-inverting

input and set the output polarity bit in the

comparator control register to invert the

comparator output.

Note: Typical propagation delay for the

circuit is 250-350 ns using the typical

on-chip comparator peripheral of a

microcontroller.

Figure 16-1: Inverter

VDD

PIC

Microcontroller Comparator Tips ‘n Tricks

TIP #17 Logic: AND/NAND Gate

This tip shows the use of the comparator to

implement an AND gate and its complement

the NAND gate (see Figure 17-2). Resistors R1

and R2 drive the non-inverting input with 2/3 the

supply voltage. Resistors R3 and R4 average

the voltage of input A and B at the inverting

input. If either A or B is low, the average voltage

will be one half V and the output of the dd

comparator remains low. The output will go

high only if both inputs A and B are high, which

raises the input to the inverting input above 2/3

V . dd

The operation of the NAND gate is identical

to the AND gate, except that the output is

inverted due to the swap of the inverting and

non-inverting inputs.

Note:

Typical propagation delay for the

circuit is 250-350 ns using the typical

on-chip comparator peripheral of a

microcontroller. Delay measurements

were made with 10k resistance

values.

While the circuit is fairly simple, there are a few

requirements for correct operation:

1. The inputs A and B must drive from ground

to V for the circuit to operate properly.dd

2. The combination of R1 and R2 will draw

current constantly, so they must be kept

large to minimize current draw.

3. All resistances on the inverting input react

with the input capacitance of the comparator.

So the speed of the gate will be affected by

the source resistance of A and B, as well as,

the size of resistors R3 and R4.

4. Resistor R2 must be 2 x R1.

5. Resistor R3 must be equal to R4.

Figure 17-1: AND Gate

VDD

Figure 17-2: NAND Gate

VDD

Example:

• Vdd = 5V, R3 = R4 = 10k

• R1 = 5.1k, R2 = 10k

PIC

Microcontroller Comparator Tips ‘n Tricks

TIP #18 Logic: OR/NOR Gate

This tip shows the use of the comparator to

implement an OR gate, and its complement, the

NOR gate.

Resistors R1 and R2 drive the non-inverting

input of the comparator with 1/3 V . Resistors dd

R3 and R4 average the voltages of the inputs

A and B at the inverting input. If either A or B

is high, the average voltage is 1/2 V and the dd

output of the comparator is high. Only if both A

and B are low does the average voltage at the

non-inverting input drop below 1/3 the supply

voltage, causing the comparator output to go

low. The operation of the NOR gate is identical

to the OR gate, except the output is inverted

due to the swap of the inverting and

non-inverting inputs.

Note:

Typical propagation delay for the

circuit is 250-350 ns using the typical

on-chip comparator peripheral of a

microcontroller. Delay measurements

were made with 10k resistance

values.

While the circuit is fairly simple, there are a few

requirements for correct operation:

1. The inputs A and B must drive from ground to

Vdd for the circuit to operate properly.

2. The combination of R1 and R2 will draw

current constantly, so they must be kept large

to minimize current draw.

3. All resistances on the inverting input react

with the input capacitance of the comparator,

so the speed of the gate will be affected by

the source resistance of A and B, as well as

the size of resistors R3 and R4.

4. Resistor R1 must be 2 x R2.

5. Resistor R3 must be equal to R4.

Figure 18-1: OR Gate

VDD

Figure 18-2: NOR Gate

VDD

Example:

• Vdd = 5V, R3 = R4 = 10k

• R1 = 10k, R2 = 5.1k

PIC

Microcontroller Comparator Tips ‘n Tricks

TIP #19 Logic: XOR/XNOR Gate

This tip shows the use of the comparator to

implement an XOR gate and its complement the

XNOR gate.

The operation is best described in three

sections:

1. Both A and B inputs are low

With both inputs low, the inverting input

is held at .7V and the non-inverting is held

at ground. This combination results in a low

output.

2. Both A and B inputs are high

With both inputs high, the inverting input is

pulled up to V and the non-inverting input dd

is equal to 2/3 V (the average of V inputs dd dd

and G ). This combination also results in a nd

low output.

3. Input A or B is high

With one input high and one low, The

inverting input is held at .7V and the

non-inverting input is equal to 1/3 V (the dd

average of a V input and G ). This dd nd

combination results in a high output.

Note: Typical propagation delay for the

circuit is 250-350 ns using the typical

on-chip comparator peripheral of a

microcontroller. Delay measurements

were made with 10k resistance

values.

While the circuit is fairly simple, there are a few

requirements for correct operation:

1. The inputs A and B must drive from ground to

Vdd for the circuit to operate properly.

2. All resistances on the both inputs react with

the input capacitance of the comparator, so

the speed of the gate will be affected by the

source resistance of A and B, as well as, the

size of resistors R1, R2, R3 and R4.

3. Resistor R1, R2 and R3 must be equal.

4. Resistor R4 must be small enough to

produce a 1.0V, or lower, voltage drop across

D1 and D2.

Figure 19-1: XOR Gate

VDD

Comparator

Figure 19-2: XNOR Gate

VDD

Comparator

Example:

• D1 = D2, = 1N4148

• R4 = 10k, R1 = R2 = R3 = 5.1k

Product specificaties

Merk:	Microchip
Categorie:	Niet gecategoriseerd
Model:	PIC24FJ64GL302

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