driver level support for re-init via power manager functions of sx1509b gpio expander

[Loubear111]

On the Thingy52, in order to reach an optimal low power current of ~8uA we must be able to turn on and off the power line to the sx1509b gpio expander.

The Thingy:52 has support in Zephyr so most peripheral setup is easy. One place where I hit a snag however was when I started to try and minimize power usage. Trying the two steps below I was able to get the average power draw to 0.66 mA while advertising BLE running a custom version of the ANCS Peripheral example (special thanks to https://devzone.nordicsemi.com/nordic/nordic-blog/b/blog/posts/optimizing-power-on-nrf53-designs):

Disabling logging: 2.5 mA -> 1.2 mA
Disable unused peripherals via device overlay: 1.2 mA -> 0.66 mA

However this was still too much power draw. I found that switching off the VDD power switch via the VDD_PWR_CTRL pin on the Thingy52 board allowed me to get the low currents I was expecting (around ~40 μA if advertising. ~8 μA if not advertising):

The nrf52832 on the Thingy:52 is powered directly from the regulator (VREG), however most other on-board peripherals are powered by VDD. This means I need to switch on VDD when I want to use another peripheral like the on-board humidity and temperature sensor. The problem here is that Zephyr initializes all peripherals on boot and has no way of re-initializing a peripheral if its power gets turned off and on again like this situation.

I was able to work around this issue by adding Power Manager code into the device drivers, that way using a peripheral would look like this:

1. Turn on VDD
2. Initialize peripheral with PM function: pm_device_action_run(temp_device, PM_DEVICE_ACTION_RESUME)
3. Use the peripheral
4. Turn the peripheral off with PM function: pm_device_action_run(temp_device, PM_DEVICE_ACTION_SUSPEND)
5. Turn off VDD

When the peripheral driver receives the PM_DEVICE_ACTION_RESUME event, it calls any required initialization code.

Is this approach correct or is a simpler solution I am missing here? Considering a few Thingy:52 peripheral drivers I looked at inside Zephyr, none of them seemed to have any Device Power Management code implemented for handling entering and leaving low power modes.

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