Let's suppose that I have two AVRs connected to one ISP connector.
To programm only one MCU in this configuration I neet to break one (or mayby several) lines goes to the MCU I don't want to be programmed in this moment.
The first line comes to mind is VCC, which would be great as I'd save power. But as we know – power will go through digital lines wich have protection diodes to VCC and GND.
So I'd choose RESET line. Any other ideas?
Best Answer
break MOSI because that's an output back to the host and break SCK so the other MCU can't see the programming commands.
edit - this still won't work as reset will be pulsed on and off during the programming. ther other data lines also need to be broken, or reset for the other MCU(s) held low.
I guess that's why most designs use a separate programming header for each MCU.