Helios64: Added Scheduled Power ON using RTC

docs/helios64/rtc.md

@@ -14,3 +14,72 @@ To save time information and allow the RTC to keep running while system is power
 
 !!! Note
     The polarity of the battery holder is indicated on the PCB with **'+'** signs.
+
+
+## Scheduled Power On using RTC
+
+User can set up a scheduled power on using RTC alarm.
+
+### Use SYSFS
+
+Run following command to check whether there is any alarm set,
+
+```bash
+cat /sys/class/rtc/rtc0/wakealarm
+```
+If nothing return, it means no alarm set.
+To reset/disable the alarm, run
+
+```bash
+echo 0 | sudo tee /sys/class/rtc/rtc0/wakealarm
+```
+
+The alarm only accept Unix epoch time. We can use *[date](https://linux.die.net/man/1/date)* utility as helper to get epoch time of our calendar.
+
+To set alarm from absolute calendar time, run
+
+```bash
+echo `date '+%s' -d '20 December 2020 02:14:10 PM'` | sudo tee /sys/class/rtc/rtc0/wakealarm
+```
+You can also set alarm from relative time using this command,
+
+```bash
+echo `date '+%s' -d '+ 1 hour 2 minutes 10 seconds'` | sudo tee /sys/class/rtc/rtc0/wakealarm
+```
+
+After alarm set, you can power off the system and keep the power plugged in. Helios64 should automatically power on at the scheduled time.
+
+### Use rtcwake
+
+Run following command to check whether there is any alarm set,
+
+```bash
+sudo rtcwake -m show
+```
+
+To reset/disable the alarm, run
+
+```bash
+sudo rtcwake -m disable
+```
+
+To set alarm from absolute calendar time, run
+
+```bash
+sudo rtcwake -m off --date '2020-12-20 14:14:10'
+```
+
+You can also set alarm from relative time using this command,
+
+```bash
+sudo rtcwake -m off --date '+ 1 hour 2 minutes 10 seconds'
+```
+
+After the command successfully executed, system will shutdown. Keep the power plugged in and Helios64 should autommatically power on at the scheduled time.
+
+## References
+
+[date- Linux manual page](https://linux.die.net/man/1/date)
+
+[rtcwake - Linux manual page](https://linux.die.net/man/8/rtcwake)
+