Power-up sequencing

As there is a significant amount of power needed for the synth, it may be a good idea to not turn on everything at once.

Fortunately, the DC-to-DC converters have CTRL-pins on them. Setting this to GND disables the output, and setting a logic high turns on power.

I have not found the exact voltage needed for logic high on the converters I use (YLPTEC URA2412YMD-20WR3), but elsewhere I've found 3.5V for similar converters. Testing with direct connection to the 3v3 output pins on the Teensy was successful, but I'm still a bit skeptical about doing this "in production", in case it doesn't work.

The solution is to level shift the signals. At the same time, I don't want to invert the signal, so a simple transistor level shifter would require multiple parts.

My solution: Use TTL-compatible CMOS, the 74HCT series. By connecting the supply to 5V, the output logic will be 5V. At the same time, the input is 3v3 compatible. I've tested this with a 74HCT08 quad AND gate and it works perfectly.

Adding 4.7k resistors to the output makes sure the output stays at GND as long as the power is off (? these are not tristate outputs, so not sure this is a good or even needed solution. Perhaps a pulldown on the input instead or in addition is better) 

 I've ordered two cmos chips for testing:

74HCT164, a octal output shift register - this lets me control all voice card power using two pins of the microcontroller. There is no inhibit though, so the outputs have to be turned on in sequence

CD4504 - hex level shifter, with dual supply pins. 

We'll see what feels best later. I guess a power distribution board with some logic on it will be worth considering.