I have an atmega8 with a external oscillator (=crystal?!) at 8MHz running on a pcb. As long as the atmega uses the internal clock everything works fine. Then I wanted to change the fuses to work with the external osc. The guide I was following suggested to use 0xDF for the high fuse and 0xFF for the low fuse. I tested these settings with http://www.engbedded.com/fusecalc and thats external crystal startuptime +64ms and SPIEN enabled. Sounds right for me, so I used avrdude to set these fuses. Now (some hours later) I wanted to load another program onto the chip and I am not able to get a connection. Only timeouts. The program on the chips seems to work though (even after restarts).
Any idea what could be wrong? Are the fuse settings ok?