LPC810 is a tiny ARM Cortex-M0+ microcontroller in DIP package with 4 kB flash and 1 kB SRAM, currently selling individually for $1.35 at Mouser.
This is some test code and notes relating to it.
Build using official pre-built arm-none-eabi GCC toolchain.
Memory map taken from dwelch67/mbed_samples.
lpc21isp programmer available in Homebrew.