Looks like the latest version doesn't use the XOR gate, everything done in software.
Although looking at the source code (Digital_Feed_7e2), for the life of me I can't understand how the INT0 interrupt handler works. The interrupt seems to be set to be triggered when one of the encoder channels (pin 21) is LOW, which is quite unusual as the handler will keep getting triggered unless the interrupt is disabled. Either I'm reading the code wrong, or missing something (the moment where the interrupt handler is disabled). Hopefully someone can explain this :)
