Datasheet in PDF form for the Atmel AT89C2051, a 20-pin 8051-family microcontroller with 2 KB of flash, 128 bytes of RAM and a built-in analog comparator on P1.0 and P1.1. It usually includes pin descriptions, electrical limits, timing and the programming algorithm. Technicians consult it for small controllers in clocks, counters, remote modules and hobby kits, verifying supply, oscillator and reset when a unit is dead. A dedicated programmer is needed for writing, and the chip marking should be checked against the sheet first.
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.
How to open a .pdf file
The standard format for schematics, block diagrams, power-sequence charts and service manuals.