PIC24FJ64GA104 FAMILY
9.3
Doze Mode
9.4
Selective Peripheral Module
Control
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,
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.
while using
communications completely.
a
power-saving mode may stop
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:
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.
• 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.
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.
Both bits have similar functions in enabling or disabling
its associated module. Setting the PMD bit for a module
disables 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 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.
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.
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 consump-
tion, 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.
2010 Microchip Technology Inc.
DS39951C-page 119
MICROCHIP [ MICROCHIP ]
