I've just put together a prototype board using an ATMega644 and was able to program it once with my JTAG MKII. After programming it, I was trying to connect to it to modify the fuses to work with an external crystal. The JTAG MKII will not connect anymore to do this... and if I try to program again it will not connect either. Basically no attempt at connecting works anymore... even though it initially allowed me to program the device including EEPROM. I have it set to use the external reset and am holding external reset low during connect... I've done this several times in the past now with almost identical board layouts. This was just a slightly modified board that I spent all weekend putting together and now won't let me program it again?@? It just asks if I want to switch to SPI instead, and I tell it no... then it asks if I want external reset enabled... I tell it yes and am holding the reset pin to ground... as I've done many times in the past... but it will no longer connect. I've tried to erase the chip and it won't do that without connecting either... I'm totally at a loss at this point.
Does anyone have any ideas? If I was never able to connect, I'd assume maybe I just have something wired wrong... but it allowed me to program it one time... and then just never again. My code disables JTAG in its initialization... could the connection not be working because of this? I'm thinking maybe I should've changed the crystal fuse settings to not do the divide by 8 and tell it I'm using a 16 Mhz crystal before loading my program? But now that I've loaded the program first seem to be stuck with my JTAG no longer connecting no matter what I do...