Datasheet for the Atmel AT89C55, an 8051-family microcontroller with 20 KB of reprogrammable flash and 256 bytes of RAM, supplied as a 421 KB PDF. It generally includes pin assignments, the memory map, timers, serial port, interrupt structure and the parallel programming algorithm. Technicians refer to it when diagnosing legacy control boards, checking oscillator, reset and EA pins, or when sourcing a replacement such as the later AT89C55WD. Verify the package marking and read out existing code before erasing any chip.
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