The Touch MCU board contains the AVR microcontroller used for the actual touch acquisition. The touch channels are connected to a card-edge connector for connection to a touch sensor board. The SPI and TWI lines are made available on a standardized 10-pin connector for easy connection to the QT600 USB Bridge.
See schematics for the QT600 MCU boards:
QT600-ATTINY88-QT8.pdf
QT600-ATXMEGA128A1-QT16.pdf
QT600-ATMEGA324-QM64.pdf
The MCU must be programmed with a firmware containing the QTouch library, a SPI Master or TWI Master driver, and the protocol layer for the touch data packets. Complete example projects for the three QT600 MCU boards can be downloaded from http://www.atmel.com/QT600.