PIC16C558-04/P - Microchip Technology

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 NOP and Idle Mode ...................... 2-9

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 µF

33 pF

10k

22 pF

100k

0.1 µF

3.3V

Serial EEPROM

GND

VCC

SCL

SDA

RB0/INT

RB1

RB2

RB3

RB4

RB5

RB6

RB7

VDD

RA0

RA1

RA2

RA3

RA4/TOCKI

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 µF

33 pF

32.768 kHz

33 pF

10k

PIC16F819

RB0/INT

RB1

RB2

RB3

RB4

RB5

RB6

RB7

VDD

RA0

RA1

RA2

RA3

RA4/TOCKI

OSC1/CLKIN

OSC2/CLKOUT

VSS

MCLR

100k

22 pF

100k

0.1 µF

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 Vd d or Vs s .

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 Vd d or Vs s ). Both

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 Vd d

or Vs s . If the input voltage is near the midpoint

between Vd d and Vs s , the transistors inside the

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 Vd d and Vs s . Sometimes it

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, I2C™

signals, switches and for resistor dividers. For

example, a typical I2C pull-up is 4.7k. However,

when the I2C 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 I2C 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,

Vd d , is a useful step to reduce the overall

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 Vd d is not constant, a regulator

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 Vd d caused by the diode turn-on

delay when switching supplies, ensure enough

decoupling capacitance is used on Vd d (C1).

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:

Tc y

(Instruction Time) Instruction

ORG 0x05 ;Reset vector

125 ms @ 32 kHz BSF STATUS,RP0 ;bank1

125 ms @ 32 kHz BSF OSCCON,IRCF2 ;switch to 1 MHz

4 ms @ 1 MHz BSF OSCCON,IRCF1 ;switch to 4 MHz

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

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 Vd d change. Timer1/SOSC can

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

(Fo s c /4) and Timer1 has a period of 30.5 µs

(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 ‘1’ and executing the

SLEEP instruction. In PIC24, dsPIC® DSCs,

and PIC32 devices, Idle mode can be entered

by executing the instruction “PWRSAV #1”. Idle

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 NOP and Idle Mode

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 NOPs into the

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 NOPs.

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.