For the external Clock-Source I opted for a used GPS-Disipled Oscillator: The U-Blox EVK-6T Eval kit.

With the cumbersome Windows configuration program "u-center" (which works on wine ;) ) it can be set to output up to one Megaherz from the "lock" timepulse. Which is coincidentally exactly the resolution that the internal counter of the RP2040 also has, which is nice.

Of course, setting up an interrupt callback for 1MHz input would saturate the CPU considerably (in contrast to the Tuning-Fork measurement, which is a slow 0.000440MHz.

As I wanted to learn about the PIO part of the RP2040 anyway, and did not find a pulsecounter in the examples, I implemented it in a PIO, which probably could count frequencies up to 50Mhz (depending on the implementation and base clock).

To reduce load on the memory bus, I did not use a DMA transfer to read the current count from the PIO FIFO, but instead opted for a "read request", so that the PIO program only sends the current counter if the input FIFO contains a "request".
For an "on average" correct pulse count, it checks for that request on both the high and low pulse (see https://github.com/Cirromulus/tuning-fork-clock/blob/main/lib/pulsecounter.pio).

BTW, the rp2040 pio simulator did help me with debugging, especially with the limited jump conditions.

Anyway, it counts :)

I needed to add a filtering-capacitor to the fork input, as I noticed the Display heavily distorting the fork input signal if the USB-Hub is weak and has to power both the Tuning Fork Clock as well as the GPS receiver.

Now, on to logging some time with the GPS-Precision of the external clock source :)