Datasheet for the Atmel AT89S8252, an 8051-family microcontroller with 8 KB of downloadable flash, 2 KB of EEPROM and an SPI interface used for in-system programming. The PDF generally covers pin descriptions, the memory organisation, timers, serial port, watchdog and programming algorithms. It helps when replacing a failed controller or reprogramming boards in industrial controllers and test equipment. Note that firmware protected by lock bits cannot be read back, so obtain a source image and check the package marking before working.
The button opens the matching file on RM Schematics, or a source page if it is not catalogued there yet. The original attachment is from @datasheet. Enginerys does not host this file.
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The standard format for schematics, block diagrams, power-sequence charts and service manuals.