Linux/ACPI
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Last update: 2006-01-07

Documentation: Real Time Clock Alarm

Warning: /proc/acpi is being replaced by interfaces in /sys and new software should not look in /proc.

If implemented, the Real Time Clock (RTC) alarm must generate a hardware wake event when in the sleeping state. The RTC can be programmed to generate an alarm. An enabled RTC alarm can be used to generate a wake event when the system is in a sleeping state. ACPI provides for additional hardware to support OSPM in determining that the RTC was the source of the wake event: the RTC_STS and RTC_EN optional bits.
If the RTC_STS and RTC_EN bits are not supported, OSPM will attempt to identify the RTC as a possible wake source; however, it might miss certain wake events. If implemented the RTC wake feature is required to work in the following sleeping states: S1-S3. S4 wake is optional and supported through the RTC_S4 flag within the FADT (if set, then the platform supports RTC wake in the S4 state).
The user-space interface can be found in GNU/Linux at /proc/acpi/alarm.

Operation Command Sample Output
Read cat alarm  
Write echo yyyy-mm-dd hh:mm:ss > alarm
where the characters denote the appropriate elements of a date, for instance:
echo 2004-02-22 15:00:00 > alarm
Afterwards you might put the system to sleep with:
echo 5 > /proc/acpi/sleep
but be careful to stop any services, file systems, etc. before powering off, e.g. use /sbin/halt instead.