[sdiy] Why is TB-303 IC6 connected to both digital and analogvoltage rails?

[Colin f]

> I am told that one of the schmitt triggers in IC6 is is a 
> power supervisor - it keeps the CPU in reset until the 
> digital power supply is up. Does that sound correct?

It not only sounds correct, you can see the pin 4 output from IC6 running to
the reset input (pin 7) on the CPU.
 
> The 5.333V supply also powers IC9 (hex type D flip-flop, VCO 
> CV DAC latch), IC11 (dual op-amp, VCO), and IC13 (dual type D 
> flip-flop, envelope).

The CPU needs to be sure the logic on the 5.333V rail is active before it
starts writing to it.
So it's held in reset until that rail is ready.

Cheers,
Colin f