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please I am beginner ..
when I want to program the atmega-8L on the programmer ,the message apeares , it requires the calibration byte .
what is the byte calibration and what is its value.
Presumably it's similar to the calibration word in small PIC's?, as such it's stored inside the chip, placed there during testing at the factory. Your programmer 'should' automatically read and restore the calibration byte as you erase or write the chip - it's actual value will vary depending on the specific chip, that's why it's called a 'calibration' byte.
The datasheet for the device should tell you where it's stored, for PIC's it's the highest word in program memory.
Are you refering to the signature byte? The calibration byte is not required for programming, perhaps you're refering to the signature byte? In which case your programmer is telling you you've set it up to program the wrong type of device. What hardware and software are you using for programming ? On the AVR series the configuration bytes are read only as they're OTP fuses tripped during manufactoring, I've heard of a few issolated cases of these fuses becoming corrupted but it is extremely rare.
Have you looked through the ATmega8 Data Sheet? There should be section on calibration of the internal clock, and pretty much anything else. I've only done Attiny13L and Attiny2313L, haven't calibrated anything. Believe there is an Appnote on the Atmel website concerning this as well.