Jeff Epler over at Adafruit has written an excellent guide to using the Pico’s PIO functionality in combination with CircuitPython. As he points out, CircuitPython sometimes interacts differently with the PIO than standard MicroPython, so this guide was necessary to highlight the differences. It explains things in detail and gives several LED and NeoPixel-related examples. The guide can be found here and the API documentation can be found here and here.