In order to calculate where the finger touched the surface, I needed a powerful enough microcontroller. It also had to support USB in order to be able to communicate to a computer. I am planning to emulate an HID mouse with the microcontroller. I had a PIC18F4550 lying around with seems to fit to the requirements : it supports USB and has plenty of RAM !
But I cannot connect the piezos directly to the microcontroller, I first need to process the signals and make them a little more readable !