Atmel datasheet for the AT89S53, an 8051-compatible microcontroller with 12 KB of in-system programmable flash and SPI serial programming. This PDF typically supplies the pinout for the available packages, special function registers, timers, watchdog and the programming interface timing. Technicians use it to identify reset, crystal and ISP pins when a controller does not start or needs new firmware. The part is long discontinued, so the document also helps in judging pin-compatible replacements; verify ratings and memory size before substituting.
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