pio_blink.py and pio_irq.py examples uses 1000Hz frequency which is too low for Pi Pico

[kevinjwalters]

The examples https://github.com/raspberrypi/pico-micropython-examples/blob/master/pio/pio_irq.py and https://github.com/raspberrypi/pico-micropython-examples/blob/master/pio/pio_blink.py set a frequency of 1000Hz using freq=1000.

If my understanding is correct then the maxium divisor is 65535 + 255/256 which at typical 125MHz gives 1907.349Hz as the lowest achievable frequency. That makes these examples misleading as that frequency cannot be achieved and is almost wrong by a factor of 2.

Is 2000Hz as used by https://github.com/raspberrypi/pico-micropython-examples/blob/master/pio/pio_1hz.py a better choice for low frequency for examples? That's precisely attainable at 125MHz and close at 133Mhz (1.47% over).

[richardm292]

Just checking as I my have raised a duplicate issue...

I though the clock gets divided by 65535 first and then was further adjusted by the fractional part allowing ( still very approximate) lower clock counts.

2000 is still the lowest accurate frequency.

More reading for me to do.

[lurch]

@kevinjwalters Yes, I've just been looking into this and if you request a PIO frequency of 1000Hz I believe you'd actually get a frequency of 2102.1 Hz (see my comment on micropython/micropython#7025 for a demonstration of how the maths works). So changing them to actually run at 2000Hz will probably have very little noticeable impact on their output 🙂

[kevinjwalters]

My point wasn't about exact accuracy it was about providing example code which uses a value below the lower limit which is misleading as developers will see that value and think it's achievable.

[lurch]

Yup, which PR #26 fixes...