Planck keyboard powered by STM32F103C8T6
- Cheap $3 controller
- ARM M3
- 32 io pins
- TMK firmware
ARM based Teensy's. 10000 scan cycles per second. This is with the default 30us delay. It can probably go even faster.
These require a 3.3v TTL serial adapter to program them. You can also get these for a few dollars on eBay.
I soldered the serial port pins and the boot jumper/reset pins to a piece of SIP header so they can be accessed when the keyboard is assembled.
The major downside of these boards are the size and the height of the components. The jumper headers are the tallest parts but can be removed. There are also components on the bottom of the board so it does not lay flat.