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Compiled Tips ‘N Tricks Guide

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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

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 #16 Optimizing Destinations

• Destination bit determines W for F for result

• Look at data movement and restructure

Example 16-1

Example: A + B A→

MOVF

ADDWF

MOVWF

MOVF

ADDWF

A,W

B,W

A,F

3 instructions 2 instructions

Careful use of the destination bits in instructions

can save program memory. Here, register A and

the A register. A destination option is available

for logic and arithmetic operations. In the rst

example, the result of the ADDWF instruction is

placed in the working register. A MOVWF

instruction is used to move the result from the

working register to register A. In the second

example, the ADDWF instruction uses the

destination bit to place the result into the A

TIP #17 Conditional Bit Set/Clear

• To move single bit of data from REGA to

REGB

• Precondition REGB bit

• Test REGA bit and x REGB if necessary

Example 17-1

BTFSS

BCF

BTFSC

BSF

BCF

BTFSC

BSF

REGA,2

REGB,5

REGA,2

REGB,5

REGA,2

REGB,5

4 instructions 3 instructions

One technique for moving one bit from the

REGA register to REGB is to perform bit tests.

In the rst example, the bit in REGA is tested

using a BTFSS instruction. If the bit is clear,

the BCF instruction is executed and clears the

REGB bit, and if the bit is set, the instruction

is skipped.The second bit test determines if

the bit is set, and if so, will execute the BSF

and set the REGB bit, otherwise the instruction

is skipped. This sequence requires four

instructions.

A more efcient technique is to assume the

bit in REGA is clear, and clear the REGB bit,

and test if the REGA bit is clear. If so, the

assumption was correct and the BSF instruction

is skipped, otherwise the REGB bit is set.

The sequence in the second example uses

three instructions because one bit test was not

needed.

One important point is that the second example

will create a two-cycle glitch if REGB is a port

outputting a high. This is caused by the BCF

and BTFSC instructions that will be executed

regardless of the bit value in REGA.

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 #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

TIP #14 Use and Idle ModeNOP

When waiting on a blocking loop (e.g. waiting

for an interrupt), instead put the device into

Idle mode to disable the CPU. The peripheral

interrupt will wake up the device. Idle mode

consumes much less current than constantly

reading RAM and jumping back. If the CPU

cannot be disabled because the loop required

some calculations, such as incrementing a

counter, instead of doing a very tight loop

that loops many times, add s into the NOP

loop. See the code example below. A NOP

requires less current to execute than reading

RAM or branching operations, so current can

be reduced. The overall loop count can be

adjusted to account for the extra instructions for

the s.NOP

Example:

Replace:

while(!_T1IF);

with Idle mode:

IEC0bits.T1IE = 1;

Idle();

and replace:

while(!_T1IF){

i++;

}

with extra NOP instructions:

while(!_T1IF){

i++;

Nop();

}

TIP #15 Peripheral Module Disable

(PMD) Bits

PIC24, dsPIC DSCs, and PIC32 devices

have PMD bits that can be used to disable

peripherals that will not be used in the

application. Setting these bits disconnects

all power to the module as well as SFRs for

the module. Because power is completely

removed, the PMD bits offer additional power

savings over disabling the module by turning

off the module’s enable bit. These bits can be

dynamically changed so that modules which are

only used periodically can be disabled for the

remainder of the application. The PMD bits are

most effective at high clock speeds and when

operating at full speed allowing the average

power consumption to be signicantly reduced.

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.

Product specificaties

Merk:	Microchip
Categorie:	Niet gecategoriseerd
Model:	PIC24FJ64GA406

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