Crust
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As per the README.md on the crust github:
Crust improves battery life and thermal performance by implementing a deep sleep state. During deep sleep, the CPU cores, the DRAM controller, and most onboard peripherals are powered down, reducing power consumption by 80% or more compared to an idle device.
For this to work, Crust runs outside the main CPU and DRAM, on a dedicated always-on microprocessor called a System Control Processor (SCP). Crust is designed to run on a specific SCP implementation, Allwinner's AR100.
To build crust manually with uboot you can find instructions Here.
RTC wakeups
It is possible to set the device to wakeup at a specific time or after a certain number of seconds using rtcwake:
rtcwake -m mem -s 10
This example will put the device to sleep for 10 seconds and then wake it up. More information on the rtcwake command can be found in the man pages
Manual suspend
For manually suspending the device on distributions with Systemd you can use the following command with super user permissions:
systemctl suspend
On non-systemd distributions you can directly echo:
echo mem > /sys/power/state