Datasheet

 2013-2014 Microchip Technology Inc. DS70000689D-page 155
dsPIC33EPXXXGM3XX/6XX/7XX
10.3 Doze Mode
The preferred strategies for reducing power consumption
are changing clock speed and invoking one of the power-
saving modes. In some circumstances, this cannot be
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 can introduce
communication errors, while using a power-saving mode
can 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
continues 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 configu-
rations, from 1:1 to 1:128, with 1:1 being the default
setting.
Programs can use Doze mode to selectively reduce
power consumption in event-driven applications. 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. An automatic return to full-speed CPU
operation on interrupts can be 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 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, using 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 do not have effect and read
values are invalid.
A peripheral module is enabled only if both the
associated 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 one
instruction cycle. Similarly, if a PMD bit is
cleared, the corresponding module is
enabled after a delay of one instruction
cycle (assuming the module control regis-
ters are already configured to enable
module operation).