[Microbit-Python] Which pins can cope with a 35 micro second pwm?

[Andrew Stacey]

I'm not quite clear from the docs which pins can cope with the minimum 
pwm speed.  I just tried with pin1 and couldn't set it below 1000 
microseconds, but I notice from the relevant doc page 
(https://microbit-micropython.readthedocs.io/en/latest/pin.html) that 
only some pins are listed as "analog" (3,4,10).  Is it only those that 
can go down to 35?

Thanks,
Andrew