<avr/sleep.h>: Power Management and Sleep Modes -
void sleep_enable (void)
void sleep_disable (void)
void sleep_cpu (void)
Use of the SLEEP instruction can allow an application to reduce its
power comsumption considerably. AVR devices can be put into different
sleep modes. Refer to the datasheet for the details relating to the
device you are using.
There are several macros provided in this header file to actually put
the device into sleep mode. The simplest way is to optionally set the
desired sleep mode using set_sleep_mode() (it usually defaults to idle
mode where the CPU is put on sleep but all peripheral clocks are still
running), and then call sleep_mode(). This macro automatically sets the
sleep enable bit, goes to sleep, and clears the sleep enable bit.
Note that unless your purpose is to completely lock the CPU (until a
hardware reset), interrupts need to be enabled before going to sleep.
As the sleep_mode() macro might cause race conditions in some
situations, the individual steps of manipulating the sleep enable (SE)
bit, and actually issuing the SLEEP instruction, are provided in the
macros sleep_enable(), sleep_disable(), and sleep_cpu(). This also
allows for test-and-sleep scenarios that take care of not missing the
interrupt that will awake the device from sleep.
This sequence ensures an atomic test of some_condition with interrupts
being disabled. If the condition is met, sleep mode will be prepared,
and the SLEEP instruction will be scheduled immediately after an SEI
instruction. As the intruction right after the SEI is guaranteed to be
executed before an interrupt could trigger, it is sure the device will
really be put to sleep.
Some devices have the ability to disable the Brown Out Detector (BOD)
before going to sleep. This will also reduce power while sleeping. If
the specific AVR device has this ability then an additional macro is
defined: sleep_bod_disable(). This macro generates inlined assembly
code that will correctly implement the timed sequence for disabling the
BOD before sleeping. However, there is a limited number of cycles after
the BOD has been disabled that the device can be put into sleep mode,
otherwise the BOD will not truly be disabled. Recommended practice is
to disable the BOD (sleep_bod_disable()), set the interrupts (sei()),
and then put the device to sleep (sleep_cpu()), like so:
void sleep_cpu (void) Put the device into sleep mode. The SE bit must be
set beforehand, and it is recommended to clear it afterwards.
void sleep_disable (void) Clear the SE (sleep enable) bit.
void sleep_enable (void) Put the device in sleep mode. How the device is
brought out of sleep mode depends on the specific mode selected with
the set_sleep_mode() function. See the data sheet for your device for
Set the SE (sleep enable) bit.
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Version 1.6.8 <avr/sleep.h>: Power Management and Sleep Modes(3)