ACPI Wakeup

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System Wakeup


Most users tend to leave their mythTV system running 24 by 7. This page describes the options you have in Linux to shutdown and automatically wakeup your machine before a recording. A glance at your last electricity bill shows you just how expensive electricity is. You can save energy and money. [read more]

Your options
  • Hardware time switch - a cheap power switch and a simple cronjob to shutdown the machine can do magic.
  • Wake On LAN - have another machine on your network wake your mythTV machine
  • Use the BIOS - most machines have an option in the BIOS for time-controlled booting

Use the BIOS

The Advanced Configuration and Power Interface (ACPI) specification is an open industry standard first released in December 1996 developed by HP, Intel, Microsoft, Phoenix, and Toshiba that defines common interfaces for hardware recognition, motherboard and device configuration and power management.

The ACPI Real Time Clock Alarm Function can be used on your mythTV system to wake-up your machine at a certain time and/or date.

If you don't have an ACPI-compliant BIOS, you could use nvram-wakeup instead, which is a small program which modifies the BIOS settings memory (NVRAM) directly. (not recommended) All modern BIOSes should support ACPI, however.

BIOS setup

If you want to use ACPI to wake up your mythTV system, you first need to ensure that your BIOS has support to wake. Check your BIOS for the alarm function. This setting is likely found under a sub-menu of "Power Management" "APM", or "Advanced" - you just need to poke around to find it. It will likely be called something like Wake from RTC, Wake From Alarm, RTC Resume, etc.

Important.png Note: There are a lot of fussy BIOSes out there. In some cases you need to disable the RTC alarm function is the bios to make things work. After you have set the RTC alarm from Linux, it could be that you not see changes in the BIOS but it still works. This is what we call Fussy Bios.

UTC, localtime and BIOS date format

If supported by your BIOS, set the time to UTC (Coordinated Universal Time). If not supported in the BIOS you need to recalculate the time before you write it to the BIOS. (one hour difference in most cases)

Also check the format of the date and time are stored in the bios. Most likely the format is YYYY-MM-DD HH:MM:SS

Write down these values, can be useful later ;-)

Prepare your Linux distribution

The ACPI interface in Linux has changes with kernel 2.6.22. Make sure you understand what kernel version you are using:

Check your kernel version

uname -a
  • Kernel > 2.6.22 and higher use /sys/class/rtc/rtc0/wakealarm
  • Kernel < 2.6.21 and lower use /proc/acpi/alarm

Check if interface exists


Kernel > 2.6.22 and higher use /sys/class/rtc/rtc0/wakealarm

ls /sys/class/rtc/rtc0/wakealarm
format of the /sys/class/rtc/rtc0/wakealarm

1. Instead of accepting a formatted time, wakealarm accepts the number of seconds since Jan 1, 1970. (e.g. 1214837639 = Mon Jun 30 16:54)

2. If you want to change the wakealarm time, you first need to reset the time.

If you can't find /sys/class/rtc/rtc0/wakealarm, and you're running a modular kernel, it might be the case that the correct module is not loaded. In that case, make sure the rtc module is unloaded, and then load the rtc-cmos module:

rmmod rtc
modprobe rtc-cmos

  • Kernel < 2.6.21 and lower use /proc/acpi/alarm
ls /proc/acpi/alarm 
format of /proc/acpi/alarm

1. Year-month-day hour:min:sec - yyyy-MM-dd hh:mm:ss - (e.g. 2005-12-29 10:10:04)

ACPI enabled

Check if ACPI is enabled in your kernel and working.

$powersave -S

Disable hwclock updates

On most machines it's required to make a small change to the Linux shutdown procedure. When your machine goes down, most linux distributions write the system time/data back to the bios. On MANY machines the machines never wakes-up after a time/data update. It's recommended to make this change before you start. See below for more details (distro specific),

Most linux distributions write the current system time back to the bios when shutting down the machine. Some BIOSes refuse to wake up if the hardware clock is modified after the alarm timer has been set. To avoid that the current system time is written back to the hardware clock it required to change your startup scripts. This is distribution specific, so here are some examples:


Set clock_systohc to "NO" in /etc/conf.d/hwclock:

# Set CLOCK to "UTC" if your system clock is set to UTC (also known as
# Greenwich Mean Time).  If your clock is set to the local time, then
# set CLOCK to "local".  Note that if you dual boot with Windows, then
# you should set it to "local".

# If you want to set the Hardware Clock to the current System Time
# during shutdown, then say "YES" here.
# You normally don't need to do this if you run a ntp daemon.

# If you wish to pass any other arguments to hwclock during bootup,
# you may do so here. Alpha users may wish to use --arc or --srm here.
Geeko head48.png openSUSE

modifying /etc/sysconfig/clock holds a switch to write back system time to the hardware clock

==>  SYSTOHC="yes"
# Is set to "yes" write back the system time to the hardware
# clock at reboot or shutdown. Usefull if hardware clock is
# much more inaccurate than system clock.  Set to "no" if
# system time does it wrong due e.g. missed timer interrupts.
# If set to "no" the hardware clock adjust feature is also
# skipped because it is rather useless without writing back
# the system time to the hardware clock.
==>  SYSTOHC="no"

Ubuntu & Debian

modifying /etc/init.d/ with the following will fix this problem:


==>         ACPITIME=`cat /proc/acpi/alarm`
                if [ "$HWCLOCKACCESS" != no ]
                    if [ "$VERBOSE" != no ]
                        echo "Saving the System Clock time to the Hardware Clock..."
                    [ "$GMT" = "-u" ] && GMT="--utc"
                        /sbin/hwclock --systohc $GMT $BADYEAR
                    if [ "$VERBOSE" != no ]
                        echo "Hardware Clock updated to `date`."
==>              echo "$ACPITIME" > /proc/acpi/alarm
Fedora Core 6

modifying /etc/init.d/halt with the following will fix this problem:

==>  ACPITIME=`cat /proc/acpi/alarm`

     [ -x /sbin/hwclock ] && action $"Syncing hardware clock to system time" /sbin/hwclock $CLOCKFLAGS
==>  echo "$ACPITIME" > /proc/acpi/alarm

Using /proc/acpi/alarm

Before making complex scripts, do a simple test to ensure that everything works: Check if the wakeup function works: The commands set the wakeup time to 5 minute from now, regardless of whether the RTC is in UTC or locatltime and then turns off the machine.

# echo "+00-00-00 00:05:00" > /proc/acpi/alarm 
# shutdown -h now

If you're machine does not start 5 minutes later, check the following:

Here are the commands to check that you have ACPI working.

$powersave -S

Check that you have the alarm function in /proc/acpi/alarm. This prints out the RTC Alarm clock from the BIOS. (Note: If you have a 2.6.22+ kernel, see next section instead.)

$cat /proc/acpi/alarm
 2005-**-29 10:10:04

Check that you can write a new time to the RTC Clock Alarm (you will need to be Root) Format is YYYY-MM-DD HH:MM:SS

#echo "2005-12-29 10:10:04" >/proc/acpi/alarm

Check that it was written, shut down and wait for it to wakeup.

#cat /proc/acpi/alarm
 2005-*12-29 10:10:04 
#shutdown -h now

For example (Assuming bios is in localtime):

Wakeup time format: yyyy-MM-dd hh:mm:ss
Set wakeuptime command: sudo sh -c 'echo $time > /proc/acpi/alarm'

Using /sys/class/rtc/rtc0/wakealarm

Beginning with kernel 2.6.22 /proc/acpi/alarm has been removed and replaced with /sys/class/rtc/rtc0/wakealarm. The key differences with the wakealarm interface are:

  1. Instead of accepting a formatted time, wakealarm accepts the number of seconds since Jan 1, 1970.
  2. If you want to change the wakealarm time, you first need to reset the time.
  3. You have to make sure that your bios clock is set to UTC time not localtime otherwise it will wakeup at the wrong time

Simple test
echo 0 > /sys/class/rtc/rtc0/wakealarm
echo `date '+%s' -d '+ 5 minutes'` > /sys/class/rtc/rtc0/wakealarm
cat /sys/class/rtc/rtc0/wakealarm


cat /proc/driver/rtc

This should return some number. Shutdown your computer and see if it comes back up in ~5 min.

shutdown -h now 


Warning.png Warning: This part needs a better explanation, with some good samples - If you have some free cycles? Think you need to set mythtv-setup If you need seconds since the epoch (1970-01-01) set the "Wakeup time format" to "time_t".

This acpi-alarm script will function with the new sys wakealarm interface or the old acpi/alarm interface

# $1 is the --settime switch that nvram-wakeup normally expects
# $2 is the date/time in seconds since 1970

DATE=`date -d "1970-01-01 $2 sec" "+%F %H:%M:%S" -u`
SECS=`date -d "1970-01-01 $2 sec" "+%s" -u`

# Save the wakeup time
echo "$*"  > /myth.wakeup.args
echo $DATE > /myth.wakeup.time
echo $SECS > /myth.wakeup.secs

if [ -e /sys/class/rtc/rtc0/wakealarm ]; then
        echo 0 > /sys/class/rtc/rtc0/wakealarm
        echo $SECS > /sys/class/rtc/rtc0/wakealarm
if [ -e /proc/acpi/alarm ]; then
        echo $DATE > /proc/acpi/alarm

If leaving the RTC wakeup alarm on the BIOS worked with the old interface, you may need to disable it when migrating to the new interface.

Warning.png Warning: The wakealarm interface is incompatible with the kernel's old "Enhanced Real Time Clock Support" and "Generic /dev/rtc emulation" options. If your kernel was built with these enabled your kernel log will contain messages such as
rtc_cmos: probe of 00:03 failed with error -16
The solution is to rebuild your kernel with the above two options excluded (find them under Drivers -> Character Devices) and the various RTC interfaces (found under Drivers -> Real Time Clock) included. From a .config point of view CONFIG_RTC and CONFIG_GEN_RTC must be unset and, at a minimum, RTC_INTF_SYSFS must be set.


Getting autostart using alarm can be quite frustrating. Here's some tips on getting it working.

Check your hardware

Check if your BIOS supports ACPI/RTC You will need to have a motherboard that supports both ACPI and a Real Time Clock Alarm Functions in the BIOS. Check your BIOS and look for a tab similar to "Advanced Power management".

The wake-up function is provided on most modern boards by the ability to "wake from RTC alarm" - setting a date and time in BIOS will cause the computer to power itself on and boot at the set time. This setting is likely found under a sub-menu of "Power Management" "APM", or "Advanced" - you just need to poke around to find it. It will likely be called something like Wake from RTC, Wake From Alarm, RTC Resume, etc.

Initiate manually

The following example will set the wakeup time to 5 minute from now, regardless of whether the RTC is in UTC or locatltime. You may not see the change in the bios, but it should still work.

if you are using the /proc/acpi/alarm interface
echo "+00-00-00 00:05:00" > /proc/acpi/alarm 

Check that it was written, the format is YYYY-MM-DD HH:MM:SS

cat /proc/acpi/alarm

If the year part looks like "0007" instead of "2007", change +00 with +2000; Shutdown your machine and wait for it to wakeup

shutdown -h now
if you are using the 2.6.22+ rtcX interface
echo 0 > /sys/class/rtc/rtc0/wakealarm
echo `date '+%s' -d '+ 5 minutes'` > /sys/class/rtc/rtc0/wakealarm

Check that something was written

cat /sys/class/rtc/rtc0/wakealarm

Shutdown your machine and wait for it to wakeup

shutdown -h now

UTC or localtime

Remember, your bios time might be in UTC rather than local time using:

  # echo "+00-00-00 00:05:00" > /proc/acpi/alarm 

will set the wakeup time to 5 minute from now, regardless of whether the RTC is in UTC or locatltime.

Since the wake-up time given by mythtv to the mythtv wakeup command is in local time, you need to do some bash handling of the obtained time to be able to setup the wakeup time in the bios in UTC (independent of daylight savings time and such). The following bash code might help here:

echo $1\ $2 > $stamp_file
# If using mythwelcome you can try the next line instead on the one above.
#echo $1 | sed "s/T/ /" >$stamp_file
# Read the date in the locale time format and add the time-zone info to the stamp_file
datum=$(/bin/date -f $stamp_file +%F\ %T\ %z)
echo $datum > $stamp_file
# reinterpret this in utc and write to alarm
utcdatum=$(/bin/date -u -f $stamp_file +%F\ %T)
echo $utcdatum > $stamp_file
#rm -f $stamp_file
#echo $utcdatum >/proc/acpi/alarm

As you noticed the echo to the acpi-alarm is commented out by default, because it is really better to check first if everything is working correctly (time format the same on your bios and such).

An alternative script to the above is the following:

 #just log what we get as command line parameters
 echo $1 $2 $3> $stamp_file
 #I set mythtv to output the number of seconds since epoch
 #so I calculate the number of hours, minutes and seconds from
 #now the computer has to wakeup:
 sfn=$(($2 - `date +"%s"`))
 # Offset from GMT as the awk function does timezone correction.

 #and then send it to /proc/acpi/wakeup in the format we saw above
 y=`(echo $(($sfn - $tzone))|awk '{print strftime("+00-00-00 %H:%M:%S", $1)}')`
 echo "$y">/proc/acpi/alarm
 echo "$y">>$stamp_file
 echo "executed at `date`" >> $stamp_file

This works on a lot of different EPIA and ASUS Motherboards, and is not dependent on the timezone you use in the bios.

Set wakeup time two times (FUSSY BIOS)

For some motherboards, it is required that you set the wakeup time two times, otherwise it will not wake up.

echo "2005-12-29 10:10:04" >/proc/acpi/alarm && sleep 1 && echo "2005-12-29 10:10:04" >/proc/acpi/alarm

More glitches (FUSSY BIOS)

  • Another possible glitch is, that the option 'Resume By Alarm' (or whatever it is called) is set to Enabled but wake up using /proc/acpi/alarm only works if the option is set to Disabled. Sounds weird but works with some boards.

  • On many boards, when the RTC setting is enabled in the BIOS, it will wake only from a time set and saved from BIOS setup, and not from a time set outside of the BIOS setup environment - as we want. All of the boards the original author of this document needed this setting disabled to correctly wake with ACPI. This is the recommended starting point.

Additional source of info

Webpage.png -

Webpage.png -

Webpage.png -

Document.png - Very good article about wakeup, Linux Magazine

Extra Information

Activating ACPI support when compiling your own kernel

The setup of /proc/acpi/alarm is done in kernel file drivers/acpi/sleep/proc.c and controlled by the CONFIG_ACPI_SLEEP_PROC_FS flag. You can test support for you architecture by going to you kernel source and do:


Important.png Note: To enable acpi-sleep in the kernel, it is necessary to set "Support for hot-pluggable CPUs

( Processor type and features  ---> [*] Support for hot-pluggable CPUs) so that
"CONFIG_HOTPLUG_CPU=y" in the ".config" file. After this it is possible to set
([*]   Sleep States) under ACPI support.

User experience

Moosylog - March 2007: Works fine with MSI RS482M-IL and openSUSE 10.2 after I made the following change to /etc/sysconfig/clock.....SYSTOHC="yes" to SYSTOHC="no".

Turpie: Works great with the Asus M2NPV-VM. Rad: Yep, but needs a recent bios version

Murph - April 2007: Works also great with the Legend QDI Advance 5/133E.

mythwog: No problems with ECS PT890T-A. Simply wrote wake up time to /proc/acpi/alarm, halted, and system wakes up.

Fuchur - August 2007: Had to replace the wakeup script with the one from as my mythshutdown produces another time format. Otherwise it seems to work so far.

Mattwire - September 2007: Works fine with MSI K8NGM2-FID and NC6120 laptop running Ubuntu Feisty and Gutsy. Both require modification to /etc/init.d/ to save the wakeup time.

Dumdideldum - September 2007: Works with Epox 8rda+ running Ubuntu Feisty. It is important to disable the wakeup on rtc in the Bios and editing the /etc/init.d/ as described in the Ubuntu guide linked in the section below.

Pkendall - September 2007: Works fine with MSI K9NBPM2-FID running Ubuntu Feisty. As above, edit the file and make sure Bios RTC wakeup is disabled.

Jmwislez - September 2007: Problem with ASUS P5GD1 motherboard (BIOS v1.014). Writing 'echo "2007-09-09 14:15:00" > /proc/acpi/alarm' works perfectly, reading via 'cat /proc/acpi/alarm' yields the expected '2007-09-09 14:15:00', but the system doesn't start at the specified time. After a manually initiated boot, 'cat /proc/acpi/alarm' yields '2007-09-00 14:15:00', with '00' for the day.

Thewizzard - October 2007: Works with MSI K7N2 Delta-ILSR, be sure to use the script and have it set the time two times.

Spitzbub - December 2007: Works with Biostar NF325-A7, be sure to modify your /etc/init.d/ script (or equivalent) to set the wake-up time again after the --systohc, as described above under Fussy Bios.

Per Olofsson - December 2007: Works very well with Asus P5GC-VM motherboard and Debian lenny with Linux 2.6.22. I don't use the "fussy BIOS" hack since I only suspend the system instead of shutting it down.

KIB - February 2008: Shuttle XPC model SS58G2 / SS58v10, motherboard FS58V10 works with Mythbuntu 7.10, kernel 2.6.22-14. Required BIOS settings - PM Wake Up Events -> IRQ8:enabled, PCIPME:enabled, Powerup by Alarm:disabled. I use /proc/acpi/alarm interface and the patch to and is requred as above. Wake on LAN works as well - just add "ethtool -s eth0 wol umbg" to /etc/rc.local .

imcecil - April 2008: Work for Asus P5n-E SLI Gentoo 2.6.24 configured the Kernel as required above but used time_t as the wakeup time format (time since epoch) which seems to work when echoed straight to /sys/class/rtc/rtc0/wakealarm.

Cymen - April 2008: Works great for PCChips P53G but be sure to only have the rtc-cmos module loaded (not the rtc module -- if rtc module is loaded, unload both rtc and rtc-cmos and then load rtc-cmos again and the device will appear in /sys...). Blacklist the rtc module or don't compile it in (the option in under "character drivers" is the one to be excluded).