Setting right fuses for Atmega8 // timeout

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?

(deleted)

Try rebootloading with "ATmega NG whith Atmega8" as the board type. Or at least use those fuse settings. Can read them in the boards.txt file.

I used this changed atmega8 code from the boards.txt:

##############################################################

atmega8.name=Arduino w/ ATmega8 internal @8Mhz

atmega8.upload.protocol=arduino
atmega8.upload.maximum_size=7168
atmega8.upload.speed=19200

atmega8.bootloader.low_fuses=0xE4 
atmega8.bootloader.high_fuses=0xca
atmega8.bootloader.path=atmega8
atmega8.bootloader.file=ATmegaBOOT-prod-firmware-2009-11-07.hex
atmega8.bootloader.unlock_bits=0x3F
atmega8.bootloader.lock_bits=0x0F

atmega8.build.mcu=atmega8
atmega8.build.f_cpu=8000000L
atmega8.build.core=arduino
atmega8.build.variant=standard

I did upload program with an ISP and then only changed high and low fuse. No lock fuses.

How should i change settings withouth any acces now?

But the fuses are right, aren't they?

You're definitely on the correct path. I've not used any of the smaller memory or smaller chips, so I can't offer much more than to keep trying things. Perhaps go thru the steps of burning the bootloader to ensure the fuses are changed, and then then try loading the program serially again.

(deleted)

Well I did not use a bootload in that sense. Only "upload using programmer" in the ide. I changed the fuses via avrdude .. hfuse:w:xxx and so on. But now I don't even get access via avrdude, so no chance of changing anything. I just don't understand why ;(

Well thanks for your help guys, it seems to be working now. Guess it was a loose connection :cold_sweat:

Regarding the problem though: this is the right settings for the fuses, isn’t it?
I am confused though how to know which one of the “features” here http://www.engbedded.com/fusecalc if have to use. How do I know if it’s a High, medium or low frequency crystal? Where is this defined? And how do I know which startuptime I have to use?
Would be great if someone could help me here (or tell where I can read about this).