I've got a couple questions on using a secondary IC. I'm planning on using an atmega32u4 to drive my matrix and then adding an atmega328p to drive some WS2812Bs. Is it possible to program both chips without external tools? Here's what I was planning:
With MOSI, MISO, SCK, reset, power/ground, and RX/TX pins connected between the two,
1) Use the DFU bootloader on the 32u4 to upload code turning it into an AVR ISP.
2) With the above, burn a bootloader onto the 328p, then upload LED control code.
3) Upload TMK/etc to the 32u4.