Smaller PWM Code

Published on 2016-12-18 in Servo Breakout for WeMos D1 Mini.

I have been thinking about rewriting the PWM code in order to drop the inconvenient license, and possibly also bring the code under the 1kB mark, to enter it into the contest. Those are some notes from my ideas.

There are three tricks that I think could let me get this to work:

The first trick is that the servo signals don’t need to agree in phase – each of the signals is independent, so I’m completely free to decide when to start each of them. The only things that matter is the pulse width and the period of the whole signal. That means that I can generate the signals for the pins on each of the 3 ports separately – first handle the B pins, then the C pins, and then the D pins. I would still have more than enough time for two more ports, if I needed them.

The second trick is to use a timer interrupt, but set the trigger to point to the next moment in time when one or more pins need to be set low, and then move that trigger to the next such moment.

The third trick is to do most of the processing outside of the interrupt, when a servo’s position is changed – calculate the port states and delays. The interrupt should only set the registers to values it takes from arrays.

I think that this approach is simple enough to have small code (for instance it has no sorting), but fast enough to be rather accurate (the delays will probably need an adjustment for the interrupt execution time). Together with a no-interrupt i2c slave, I hope to get some pretty stable signals.