16 MHz clock, 8 KB Flash, 256 B RAM — sizing the firmware boundary
The 16 MHz CPU speed is the ceiling for the internal oscillator; you can run it slower to save current, but the core MIPS budget caps at roughly 16 MIPS. The 8 KB Flash is the entire program store — no external memory interface, so the firmware image must fit entirely in that space. The 256-byte RAM is the working set: stack, global variables, and a small FIFO. That rules out any application needing a large lookup table, a real-time OS with multiple task stacks, or a TCP/IP stack. For a single-ended UART polling loop or an I²C sensor readout, it's enough — but the linker map needs watching.
Peripherals and connectivity on a 20-pin DIP
The internal oscillator means no external crystal is needed for basic operation — useful for a minimal BOM count in a low-cost sensor node.