A 2.83 MB complete datasheet for the mega64, corresponding to the Atmel AVR ATmega64 microcontroller in a 64-pin package. No revision is stated in the name. Expect detailed pin descriptions, memory map, peripherals, fuse settings, JTAG and serial programming information, plus DC and timing characteristics. Note that ISP on this part uses dedicated pins rather than the hardware SPI pins, so the programming chapter is worth checking before wiring a programmer to a board under repair.
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.