convert an old XT (or AT) keyboard to USB using a pro micro (or teensy) with an atmega32u4

 * reset pro micro into bootloader mode: https://github.com/nicholaskell/Arduino_Loader/blob/master/reset.py
 * https://github.com/tmk/tmk_keyboard/wiki
 * https://deskthority.net/workshop-f7/how-to-use-a-pro-micro-as-a-cheap-controller-converter-like-soarer-s-t8448.html
 * https://github.com/tmk/tmk_keyboard/blob/master/tmk_core/doc/build.md
 * https://deskthority.net/workshop-f7/xt-at-ps2-terminal-to-usb-converter-with-nkro-t2510.html

tbc.