I have a rough idea on how I would use CircuitPython's asyncio capability to structure my learning project, but jumping straight into async/await in a new platform is too bold for me. I would rather ramp up gradually starting with running a few CircuitPython samples. Once I was satisfied my KB2040 board is generally working as expected, I started poking at project-specific needs. First task: see if I've soldered serial communication wires correctly by checking for activity on those pins in reaction to enable pin status. (Toggled via CircuitPython's digitalio module.) I expect activity on the receive pin when the enable pin is high, and no activity when enable is low. The transmit pin should have no activity in both cases.

I had originally intended to check pin status by polling pins in a loop, which is not guaranteed to catch all activity but probably good enough for an "is there activity" check. Polling in a loop would have been trivial in an Arduino test sketch, but CircuitPython made it just as easy to pull in something more sophisticated: countio module counts rising/falling edges driven by hardware interrupts, which would have taken a bit more work to set up on Arduino. Great! Let's do that.

I copied sample code from documentation and modified it for my purposes of watching pins D0 and D1, which were default UART transmit and receive pins on my KB2040. I saved my file and promptly got an error telling me I am not allowed to do this on D0. Huh? Re-reading countio documentation I saw a paragraph I had missed:

Limitations: On RP2040, Counter uses the PWM peripheral, and is limited to using PWM channel B pins due to hardware restrictions. See the pin assignments for your board to see which pins can be used.

Reviewing KB2040 pinout reference, I see D0 is also PWM0A and D1 is PWM0B. So they are both PWM-capable pins satisfying the first restriction, but only D1 satisfied the second restriction of channel B. Fortunately D1 on PWM0B is the default UART receive pins, so that's the one I was more interested in checking status for anyway. And the check was good: when enable pin is high, there is a steady stream of activity on D1. When enable pin is low, activity stops. Good enough to take the next step, connect UART peripheral to those pins.