Reset UART on deinit, reduces power consumption on nRF

[furbrain]

light sleep currently uses about 3-90 uA on nRF (depending on board). However, if you create a busio.UART and then deinit it, the light sleep uses about 1 mA.

This change fixes that by as per this post.

It works by using an undocumented register to power cycle the UART peripheral.

[dhalbert]

The DevZone post you link to says:

Just adding that the UARTE TX has to be stopped explicitly when it's finished. If you just close the UART without stopping the transactions the UART will continue to keep the clocks running, as described above.

This can be done through the nrfx_uarte_tx/rx_abort() functions, or the TASKS_STOPRX/TX registers directly.

If you do this before disabling the UART the above workaround is not needed.

Could we do the _abort or use the TASKS_STOP registers instead? I'd kind of rather use a documented method than rely on something undocumented whose behavior might change in a future chip rev.

[furbrain]

I absolutely agree and also feel uncomfortable using undocumented registers. However:
I did add the _abort calls but these don't seem to actually reduce current consumption on my chip - only the peripheral reset code seemed to work. I've therefore included both strategies in the PR. The RADIO peripheral on nRF52840 does also use a (documented) register at offset 0xffc for power cycling the device, so I would hope that this is in fact a general architecture decision.

[dhalbert]

Thanks for the feedback about the change. You've included a pointer to the undocumented fix, so that's the best we can do for now.