I built a small dedicated nano circuit that controls an old Noritake Itron VFD. Works great. So I wanted to expose it as a slave i2c device.
My problem is that if I power it up by supply 5V to Vcc and GND to GND, it boots (LED on) but nothing happens.
If I power it up by connecting the 5V FTDI breakout, the VFD works.
I tried removing all Serial references in case it was hanging on output, but to no avail.
What could be different in so far as being connected to a serial breakout makes it WORK?
If I plug the USB cable into a power brick it works also, so the difference seems to be either having the board connected or being powered through it, but it's not communicating with anything (as proven by working with just a power brick and not the PC connected).
Any idea are welcome!