OK, so I think this has been talked about before, and maybe even recently, but search didn't turn up anything.
If I have multiple AVR's on one board, how can I program them all with one ISP conector? Is there a standard trick? Or an Atmel ap note that I missed?
Seems to me that selecting using reset is a bad idea, since you want all the board held in reset, and any AVR in reset will go into programming mode, resulting in a muddle. My first guess at doing it correctly would be to wire up reset, mosi, and miso in parallel to all AVR's, and then route sck through a de-mux of some flavor. Routing sck through a jumper such that the device being programmed is being fed ssk and the others have a floating sck would also seem to be a bad idea, since reset disables internal pull-ups. So... a weak pull-up on everybody's sck? And then you have the issue of sck needing to go everywhere during normal spi operation.
I need a clue.