Raspberry Pi Pico's built-in bootloader allows you to write firmware via USB or SWD. However, in some situations, other interfaces may be useful. Picoboot3 now offers the options of UART, I2C and SPI.

There are smaller microcontrollers than the ATtiny13. Some ARM chips will fit on the head of a large pin, and even in Atmel world, the ATtiny10 comes in a tiny SOT-23-6 package – a size normally ...