rust-raspberrypi-OS-tutorials/0A_power

Tutorial 0A - Power management

For embedded systems, power consumption is critical. The Raspberry Pi 3 has a very sophisticated PM interface. You can turn each device on and off independently. There's a catch though. The GPIO VCC pins are hardwired, there's no way to turn them off programmatically. This means if you connect some devices to them, you'll have to implement a way to turn those devices off (with a transistor connected to a data GPIO pin for example).

power.rs

Unfortunately, the documentation about the PM interface is very very rare. We will therefore more or less implement a carbon copy of respective functions of Linux' bcm2835_wdt.c driver.

The power management controller is one of the peripherals that are not emulated properly by QEMU. Our implementation therefore works on real hardware only.

Power::off(&self, mbox: &mut mbox::Mbox, gpio: &gpio::GPIO) shuts down the board to an almost zero power consumption state.

Power::reset(&self) reboots the machine. Also handled by the PMC, and since the Raspberry Pi does not have a hardware reset button, it's very useful.

When using make raspboot and choosing reset(), you can see your code in action nicely as you generate a boot-loop.

gpio.rs

We introduce a lot of new GPIO pins. It's a good time to refactor the GPIO MMIO interface into its own type with the common RegisterBlock implementation that you already know from the other components.

main.rs

We display a simple menu, and wait for user input. Depending on the input, we reboot the system or power it off.