Complete datasheet for the ATmega164, ATmega324 and ATmega644 AVR microcontrollers, a 2.66 MB PDF. It covers pin configurations, memory sizes for each device, clock system, timers, dual USARTs, ADC, JTAG, fuses and electrical characteristics. A repair technician uses it to identify supply and programming pins, check oscillator and reset circuits, and choose correct fuse settings before writing firmware. Note the memory size differs between family members, so confirm which part is actually fitted.
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.