PIC24FJ64GA006-I Datasheet, PDF (106/258 Pages) Microchip Technology – 64/80/100-Pin, General Purpose, 16-Bit Flash Microcontrollers

PIC24FJ128GA010 FAMILY
9.2.2 IDLE MODE
Idle mode has these features:
• The CPU will stop executing instructions.
• The WDT is automatically cleared.
• The system clock source remains active. By
default, all peripheral modules continue to operate
normally from the system clock source, but can
also be selectively disabled (see Section 9.4
“Selective Peripheral Module Control”).
• If the WDT or FSCM is enabled, the LPRC will
also remain active.
The device will wake from Idle mode on any of these
events:
• Any interrupt that is individually enabled.
• Any device Reset.
• A WDT time-out.
On wake-up from Idle, the clock is re-applied to the
CPU and instruction execution begins immediately,
starting with the instruction following the PWRSAV
instruction or the first instruction in the ISR.
9.2.3
INTERRUPTS COINCIDENT WITH
POWER SAVE INSTRUCTIONS
Any interrupt that coincides with the execution of a
PWRSAV instruction will be held off until entry into Sleep
or Idle mode has completed. The device will then
wake-up from Sleep or Idle mode.
9.3 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 cir-
cumstances, 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 1:128, 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.
9.4 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, generi-
cally named “XXXMD”, located in one of the PMD
Control registers.
Both bits have similar functions in enabling or disabling its
associated module. Setting the PMD bit for a module dis-
ables all clock sources to that module, reducing its power
consumption to an absolute minimum. In this state, the
control and status registers associated with the periph-
eral 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. Power consumption
is reduced, but not by as much as the PMD bit does.
Most peripheral modules have an enable bit;
exceptions include Capture, 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.
DS39747F-page 106
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