RP2040

It launched in January 2021 alongside the Raspberry Pi Pico, a four-dollar board. Inside are two Cortex-M0+ cores at 133 MHz and 264 KB of SRAM, but no on-chip flash: the program lives in an external QSPI chip and runs from it through a cache. A small mask ROM holds a bootloader that makes the board appear as a USB drive when you hold down a button.

What sets the RP2040 apart from a hundred other microcontrollers is PIO: eight tiny state machines with their own nine-instruction ISA, able to clock out almost any protocol cycle-accurately — WS2812 LEDs, VGA, extra UARTs. It is bit-banging without the CPU having to sacrifice itself. Its successor, the RP2350 (2024), moved to Cortex-M33 and added RISC-V cores as an alternative.