1 KB Flash, 64 bytes SRAM — sizing the firmware footprint
The Microchip ATTINY13-20SQ is an 8-bit AVR MCU from the ATtiny family, built around the AVR core and clocked at 20 MHz.
20 MHz internal oscillator — no external clock needed
The internal oscillator runs the core at 20 MHz, so no external crystal or resonator is required for most applications. That saves two I/O pins and reduces BOM count. The internal oscillator is factory-calibrated; if tighter timing is needed, the part also accepts an external clock source.
A 3.7 V Li-ion cell or a 5 V USB bus can power the MCU directly.
The 105°C ceiling gives margin above the typical 85°C industrial limit, which helps with derating in sealed or high-ambient enclosures.