70592C Datasheet, PDF (138/314 Pages) Microchip Technology – High-Performance, 16-bit Microcontrollers

PIC24HJXXXGPX06A/X08A/X10A
10.2.2 IDLE MODE
Idle mode has these features:
• The CPU stops 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 10.4
“Peripheral Module Disable”).
• If the WDT or FSCM is enabled, the LPRC also
remains 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 reapplied to the CPU
and instruction execution will begin (2-4 clock cycles
later), starting with the instruction following the PWRSAV
instruction, or the first instruction in the ISR.
10.2.3 INTERRUPTS COINCIDENT WITH
POWER SAVE INSTRUCTIONS
Any interrupt that coincides with the execution of a
PWRSAV instruction is held off until entry into Sleep or
Idle mode has completed. The device then wakes up
from Sleep or Idle mode.
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 cir-
cumstances, however, where this is not practical. For
example, it may be necessary for an application to main-
tain 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 setting.
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.
For example, suppose the device is operating at
20 MIPS and the CAN module has been configured for
500 kbps based on this device operating speed. If the
device is now placed in Doze mode with a clock
frequency ratio of 1:4, the CAN module continues to
communicate at the required bit rate of 500 kbps, but
the CPU now starts executing instructions at a
frequency of 5 MIPS.
10.4 Peripheral Module Disable
The Peripheral Module Disable (PMD) registers
provide a method to disable a peripheral module by
stopping all clock sources supplied to that module.
When a peripheral is disabled via the appropriate PMD
control bit, the peripheral is in a minimum power
consumption state. The control and status registers
associated with the peripheral are also disabled, so
writes to those registers will have no effect and read
values will be invalid.
A peripheral module is only enabled if both the associ-
ated bit in the PMD register is cleared and the peripheral
is supported by the specific dsPIC^® DSC variant. If the
peripheral is present in the device, it is enabled in the
PMD register by default.
Note:
If a PMD bit is set, the corresponding
module is disabled after a delay of 1
instruction cycle. Similarly, if a PMD bit is
cleared, the corresponding module is
enabled after a delay of 1 instruction cycle
(assuming the module control registers
are already configured to enable module
operation).
DS70592C-page 138
© 2011 Microchip Technology Inc.