I use avreal32 to program my AVR uCs, and I needed to bring back to the factory defaults some fuses. Chip had CKSEL=1111 (external high-frequency XTAL) and CKDIV8=0, so effectively running at XTAL frequency. I wanted to have it back to the internal 8 MHz RC oscillator divided by 8, so: CKSEL=0010 and CKDIV8=0.
What I did was to call avreal and first change the value of CKDIV8 with:
avreal32.exe +MEGA168 -ap -p1(0x378) -o0 -4 -w+ -fCKDIV=0
Note that my original configuration on my programming hardware does not require an external XTAL for programming the uC as long as avreal32 is generating the clock with -o0 switch.
Result: chip couldn't be read, avreal32 said "Can't resync". I did not touch RSTDISABLE this time. After a bit of thinking and reading avreal32's -help I tried this command to set CKSEL:
avreal32.exe +MEGA168 -ap -p1(0x378) -o0,8 -4 -w+ -fCKSEL=2
I think that the uC was running 8 times slower than avreal32 expected it to, given the computer generated clock and the internal divider enabled. Slowing down avreal32 by a factor 8 did the trick and now I can program again the chip at my usual speed -o0.
Note that this procedure does not cure a disabled RSTDISABLE fuse. You need a HVP for that.