20 MHz AVR core in a 4 KB Flash envelope
The ATTINY44-20MU is an 8-bit AVR microcontroller from Microchip's ATtiny series, clocked at 20 MHz with 4 KB of on-chip Flash program memory and 256 bytes each of SRAM and EEPROM. The 20 MHz ceiling sets the instruction throughput — most single-cycle AVR instructions execute in 50 ns — which matters for PWM update rates and USI bit-bang timing loops.
4 KB Flash — what fits and what does not
4 KB of program memory (2K x 16) is the hard firmware budget. A typical application — reading two analog sensors over the ADC, driving a PWM output, and communicating via the USI as an I²C slave — leaves roughly 1–1.5 KB free after the core loop and peripheral init. If the firmware needs a full USB stack, a bootloader, or complex state machines, the 4 KB ceiling forces a jump to the ATtiny24 or ATtiny84 (same pinout, 8 KB / 16 KB Flash respectively).
The brown-out detection and POR peripherals ensure a clean reset on power-up and during supply dips.
The 12 I/O pins break out to the peripheral ring; the centre pad is the ground return for the core and I/O. Surface-mount reflow per J-STD-020 is standard; the MSL rating (not in evidence) should be confirmed from the Microchip package drawing before stencil design.