"Uncaught exception in IRQ callback handler" when outputing MTC at 30fps

[mungewell]

It seems we are reaching the limits of the CPU, when serving MTC at 30fps I get "Uncaught exception in IRQ callback handler".

I added a workaround, but didn't enable:
https://github.com/mungewell/pico-timecode/blob/pt_thrifty/pico_timecode.py#L334

Problem does not occur:

• when MTC is not active
• with 25fps
• with 'UART only' UF2

We may also be able to avoid by increasing CPU speed, but that would require changing the PIO clock dividers.

[mungewell]

RP2040 was re-certified for 200MHz. Looking at the 'clocks.xls' spread sheet there 'good' divider values at CPU clocked are 150MHz and 180MHz...

Perhaps this would improve situation.

[mungewell]

Modified dividers here:
https://github.com/mungewell/pico-timecode/tree/cpu_180MHz

First check and this seems to work OK, will need a bit more validation.

[mungewell]

I have not seen any more asserts since switching to 180MHz CPU clock.

Whilst testing with the 7-segment display (for a slate function), I noticed that occasionally we're not writing to the display.

I tracked this down to the 'display call-back' be serviced too late, and this means that the pt.quarters value is changed before it is assessed. This might also be affecting the MTC operation.

I removed the per-frame print() and this still occured, something else is causing a delay response.

[mungewell]

The exceptions are still occurring; when BOTH USB-console and USB-MIDI are connected and streaming.