Experimental RP2040 support using Earle F. Philhower, III Raspberry Pi Pico Arduino core

[ziotibia81]

Tested using a Raspberry Pi Pico module. This module uses a 12 MHz crystal oscillator as a reference. The clock is generated using a PLL integrated into the RP2040 CPU. The hardware timer has a fixed resolution of 1 µs/tick, so we are using an interrupt every 1000 ticks.

I was able to run SperFilter, adjusting the GPIO declarations (these are not included in this PR). I reached a synchronized state in approximately 550 seconds in a low-noise environment. I repeated this process over several days, and the result remained consistent.

Toggling an output pin each time the ISR occurs allows me to measure a frequency ranging from 497.0 Hz to 500.4 Hz. Over 30 minutes of measurements, the average (AVG) was 500.01 Hz, and the root mean square error (RMSE) was 116 mHz. When the state is 'synced', the RMSE stabilizes around 112 mHz.