Waveshare 1.54 inch mono epaper/e-ink module and Teensy (problem and answer)

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I've answered my own question - I put it here to help others who make the same mistake.

I recently bought a Waveshare 1.54 inch e-ink module and tested the sample Arduino code from their github. The results were confusingly erratic, ranging from sometimes 'not working at all' to sometimes 'random corruption of display' to sometimes 'working perfectly'. The source of power (eg USB or bench power supply) seemed to affect the results.

The problem was that I was giving the module 5 volt power and the Teensy gives 3.3V logic levels. There is a level shifter (TSX0108E) on the display module that uses Vcc to determine what logic level to expect so in my case it was trying to interpret the 3.3V signals as 5V signals. Changing the input power to 3.3V made everything work reliably.

This all now seems horribly obvious as I type it, but at the time trying to work out why it seemed to work better on the bench PSU (which I had set to 5V) than on USB (which was giving 5.1V) caused a lot of head scratching!
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