`pin.set_analog_period(0)` stops PWM output from working again

[microbit-carlos]

To replicate, in the REPL while watching pin16 in an oscilloscope or signal analyser:

>>> pin16.write_analog(512)                 # signal appears at 50 Hz
>>> pin16.set_analog_period(1)              # signal at 1 KHz
>>> pin16.set_analog_period_microseconds(1) # signal at 1 MhHz
>>> pin16.set_analog_period_microseconds(0) # signal always high
>>> pin16.set_analog_period(1)              # signal still always high

I'll try to replicate with CODAL as well and raise an issue there if necessary.

[microbit-carlos]

Raised in CODAL:

• pin setAnalogPeriodUs(0) stops PWM from working again lancaster-university/codal-microbit-v2#243