20 MHz AVR core in an 8-pin SOIC — what fits here
The Microchip ATTINY25-20SSUR is an 8-bit AVR microcontroller from the ATtiny family, qualified for Functional Safety (FuSa) applications. It runs at 20 MHz from an internal oscillator, with 2 KB of Flash program memory, 128 bytes of SRAM, and 128 bytes of EEPROM — all in an 8-pin SOIC package.
Memory budget and peripheral set
With 128 bytes each of RAM and EEPROM, this part is sized for small state machines, calibration constants, and boot counters — not data logging or large buffers. The 2 KB Flash (organized as 1K x 16) holds the firmware image; if your compiled binary exceeds that, you need a larger ATtiny or the AVR32DD series. The USI (Universal Serial Interface) can be configured for SPI or I²C, giving one external communication link.
Brown-out detection and a programmable watchdog timer are built in, so external reset supervisor ICs can often be omitted.
The Functional Safety (FuSa) qualification adds a documentation trace for safety-critical firmware development, though the part itself is not AEC-Q100 listed.