C firmware tweaks

[nilswiersma]

• Buffer last command in serial.c
• Add single character shortcuts for commands in serial.c
• Add a toggle command for GP1
• Add configurable duty_frac (not sure if that makes sense, does less duty_frac mean less voltage mean weaker pulse?), configurable pulse_time

[colinoflynn]

Thanks for PR, looks good to me! On duty cycle - having it configurable would be handy, so worth keeping in. Some quick notes on the duty cycle change:

The duty cycle is connected to cap charge voltage, but I didn't really calibrate that so I can't say how it connects. The hard-coded default was a value that seemed to work well. From memory the resulting transformer driver is closer to something like a flyback, which means it's not just e.g. a linear relationship between duty cycle and output. Going larger or smaller both results in lower charge voltage.

However going higher means more current through the transformer, which may result in some heating. Going a little higher would be fine in practice (higher meaning PWM % setting). Need to check what reasonable ranges make sense here (if it's the divider as well then I guess the divider should be increased to decrease the PWM?).

So I don't think need to change anything in your current PR for above, but may want to add some additional config eventually.

[nezza]

Thanks for the PR, looks good, I will try to test it in the next days :)

Can you remove the commented out printfs?

Thanks, Thomas

[nilswiersma]

Anything else I could do to help you test it?

[nezza]

No, thanks a lot! Sorry for the delay in testing this :)