I guess many of you were confused when programming AVR fuse bits. I get many newbie questions like “I programmed Atmega8, but it doesn’t work”. Then my standard answer is: “Did you touch configuration bits?” and if yes, then I am almost 90% sure that he did it wrong. Most of them understand wrongly that programmed fuse bit in configuration should be left unchecked and opposite. Let’s take a look at widely used programming software – PonyProg. First thing you do before programming the chip is set configuration bits (Atmega8 in the picture): The first attention should be paid to clock sources. Four bits are controlling Atmega8 clock sources: CKSEL0, CKSEL1, CKSEL2, CKSEL3.