F21 datasheet(115/416 Pages) MICROCHIP

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DS30010118D-page 115

PIC24FJ256GA705 FAMILY

10.3

Doze Mode

Generally, changing clock speed and invoking one of

the power-saving modes are the preferred strategies

for reducing power consumption. There may be

circumstances, however, where this is not practical. For

example, it may be necessary for an application to

maintain uninterrupted synchronous communication,

even while it is doing nothing else. Reducing system

clock speed may introduce communication errors,

while using a power-saving mode may stop

communications completely.

Doze mode is a simple and effective alternative method

to reduce power consumption while the device is still

executing code. In this mode, the system clock contin-

ues to operate from the same source and at the same

speed. Peripheral modules continue to be clocked at

the same speed while the CPU clock speed is reduced.

Synchronization between the two clock domains is

maintained, allowing the peripherals to access the

SFRs while the CPU executes code at a slower rate.

Doze mode is enabled by setting the DOZEN bit

(CLKDIV<11>). The ratio between peripheral and core

clock speed is determined by the DOZE<2:0> bits

(CLKDIV<14:12>).

There

are

eight

possible

configurations, from 1:1 to 1:256, with 1:1 being the

default.

It is also possible to use Doze mode to selectively

reduce power consumption in event driven applica-

tions. This allows clock-sensitive functions, such as

synchronous communications, to continue without

interruption while the CPU Idles, waiting for something

to invoke an interrupt routine. Enabling the automatic

return to full-speed CPU operation on interrupts is

enabled by setting the ROI bit (CLKDIV<15>). By

default, interrupt events have no effect on Doze mode

operation.

10.4

Selective Peripheral Module

Control

Idle and Doze modes allow users to substantially

reduce power consumption by slowing or stopping the

CPU clock. Even so, peripheral modules still remain

clocked, and thus, consume power. There may be

cases where the application needs what these modes

do not provide: the allocation of power resources to

CPU processing with minimal power consumption from

the peripherals.

PIC24F devices address this requirement by allowing

peripheral modules to be selectively disabled, reducing

or eliminating their power consumption. This can be

done with two control bits:

• The Peripheral Enable bit, generically named,

“XXXEN”, located in the module’s main control

SFR.

• The Peripheral Module Disable (PMD) bit,

generically named, “XXXMD”, located in one of

the PMD Control registers.

Both bits have similar functions in enabling or disabling

their associated module. Setting the PMD bit for a

module disables all clock sources to that module,

reducing its power consumption to an absolute mini-

mum. In this state, the control and status registers

associated with the peripheral will also be disabled, so

writes to those registers will have no effect and read

values will be invalid. Many peripheral modules have a

corresponding PMD bit.

In contrast, disabling a module by clearing its XXXEN

bit disables its functionality, but leaves its registers

available to be read and written to. This reduces power

consumption, but not by as much as setting the PMD

bit does. Most peripheral modules have an enable bit;

exceptions include input capture, output compare and

RTCC.

To achieve more selective power savings, peripheral

modules can also be selectively disabled when the

device enters Idle mode. This is done through the

control bit of the generic name format, “XXXIDL”. By

default, all modules that can operate during Idle mode

will do so. Using the disable on Idle feature allows

further reduction of power consumption during Idle

mode, enhancing power savings for extremely critical

power applications.

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