Hi all. Been messing with a half-home-made programmer so that I can finally program SMD MCUs in situ. When I power up the target board from it's normal external 12v regulated down to 5v, the programming software can the load whatever into the MCU depending on whether I am going erase, verify, program, read etc but then throws a comms error and stops before it can actually proceed with erase, verify program, read etc.
If I let the programmer supply and control the on/off of the 5v rail everything works fine. I'm happy with that except for the fact that I will need one extra wire in the programming interface for the +5v. No big deal really.
The question is though, seeing the programmer can, and does, pull the MCU reset low whenever necessary, why wouldn't this be sufficient to allow the use of the target board's continuous 5v supply?