either have a chip capable of USB or rely on a V-USB implementation, the old fogies of the Arduino world, the Uno and Mega ... between the ‘328 ISP header and the pins on the ’16u2 header.

[Rafael] just made a little library that allows you to interface these modules to the Atmega328p-based Arduino UNO in order to gain ... Our guess it that an SPI / I2C IO extender could lift ...