64-pin QFN — what the pad layout needs
The ATMEGA645P-MU comes in a 64-VFQFN with an exposed pad (9x9 mm body per the supplier device package).
16 MHz core — flash wait state and clock tree
The AVR core runs at 16 MHz from the internal oscillator.
Memory budget — 64 KB Flash, 4 KB SRAM, 2 KB EEPROM
Program memory is 64 KB of Flash (32K x 16-bit words), with 4 KB SRAM for runtime data and 2 KB EEPROM for non-volatile parameters.
Supply range and I/O tolerance
All I/O pins are 5 V tolerant when Vcc is lower — so a 3.3 V supply can safely interface with 5 V peripherals without level shifters, as long as the pin current limits are respected. The brown-out detect (BOD) can be set to trigger at 2.7 V or 4.3 V, protecting the Flash from corruption during a slow supply ramp.
On-chip peripherals include a 10-bit ADC with 8 multiplexed channels, two 8-bit and one 16-bit timer with PWM outputs, a watchdog timer, and a USART/SPI/USI serial interface.