<avr/sleep.h>: Power Management and Sleep Modes -
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
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 more details.
Set the SE (sleep enable) bit.
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