Just got a piece of the ATMEGA644-20PU PDIP format.
Able to do a blink LED program.
I changed the fuses to be 0xCF for low byte and 0x91 for high byte, so that it can work with an external crystal greater than or equal to 8Mhz, which is the 20Mhz crystal in this case. Fuses changed successfully via AVRDude.
I recompiled my blink program with F_CPU=20000000U and the program blinks the LED as if the uC running at 1 Mhz.
So, I recompiled again with F_CPU=1000000U and program blinks the LED really fast, as it should. But, how comes only with F_CPU=1000000U but not with F_CPU=20000000U?
Please explain if I have missed or done something wrong here.