Low-power options machine.freq() issue

[davefes]

Thought I'd have a go at reducing the CPU frequency to lower current consumption as per:
https://micropython-tve.readthedocs.io/en/esp32-low-power/library/esp32-lowpower.html
80MHz worked, but when I tried 40MHz I ended up with a part that to be re-flashed.

Is there some trick to get 40 and 20MHz working?