Hello!
Don't know if this is the right section but here goes.
For a project we're considering using a touchscreen shield on an arduino uno and the problem is how to use the remaining pins that the touch screen doesn't use for communication between the arduino and DSP. For SPI our DSP is only capable of acting as a master, and the UART implementation of SPI on the arduino also only works as master so that's ruled out. Is it possible to use the ICSP header to hook up the arduino to the DSP for SPI communication in parallell to the communication between touch screen and arduino? If not, is it possible to bitbang the remaining pins D0 D1 D2 D5 D6 to implement an SPI interface where the arduino acts as a slave and is this preferable to using I2C?