PDF datasheet for the Atmel AT89S51, an 8051-compatible microcontroller with 4 KB of in-system programmable flash and 128 bytes of RAM. It usually describes the pinout, memory organisation, timers, interrupt system, watchdog and SPI-based programming via the ISP pins. Use it when diagnosing a non-starting controller, checking the crystal, reset circuit and EA pin level, or when reflashing firmware with a programmer. Lock bits may block reading the existing code, so secure the correct firmware and verify the chip marking first.
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