Add support for NRF52 to the WS281x driver

[jimporter]

Description

There are currently only a couple of CPUs/architectures that support WS281x LEDs. It should be possible to add support for the NRF52, since some NRF52-based Adafruit devboards (e.g. the Feather) have a WS281x on-board, and the Arduino libs mention support for NRF52 as well.

I'm happy to work on this (and will probably tinker with it regardless of the response here), but before I go too far on this, I though it would be wise to file an issue for discussion and to hopefully steer me in the right direction.

Useful links

https://learn.adafruit.com/introducing-the-adafruit-nrf52840-feather/ (specs include a NeoPixel/WS2812 on the board)

[chrysn]

I've attempted to write a generic one based on timer and gpio_ll peripherals, and while my LEDs are blinking colorfully, I'm not convinced yet it's as stable as I'd hope it' be (especially because I had to take a few shortcuts such as hardcoding what could be a busy-loop-until-compare-is-triggered function in the timer API).

If we do need to go the per-chip route for nRF52 and similar, let's aim for something that doesn't block the CPU (similar to the ESP32 implementation that uses hardware). What would be a good peripheral to implement this on? The UARTE is too slow (maximum baud rate 1MBaud, we need 3 at least). PWM may be an option. The FastLED project uses SPI on the nRF52 (among other options); that we might manage in a platform independent way as well (provided we can get the actual clock rate back, which AIU is being worked on in #18374).

CC'ing @maribu who has also worked on this.

[jimporter]

@chrysn Yeah, a generic implementation would be ideal, assuming it's possible. SPI sounds reasonable enough to me, though I'm definitely not an expert in this area.

I see there's also #19158 for RPI Pico support which I could look at eventually too. That issue might actually have a clearer path forward since it sounds like PIO would be the ideal way to handle this.

[chrysn]

Would you mind giving #19891 a try? It's not nRF specific, but was developed on it.