I am using much of the slave code on the gammon.com SPI page, including the ISR (SPI_STC_vect). I'm using a Pro Mini for the slave and the external interrupt pins (2 and 3) are not connected to the master board. My SPI communication seems to be working fine.
Can you explain what is calling the ISR in the slave code? Is it the detection of incoming data on the MISO pin? Is this somehow handle in the SPI.h?