I have been desparately juggling the pin allocations on this project in the hope of freeing up the VREF pin, and thought I had managed to do so. Previously my system was successfully transmitting over the I2C Bus but when I move the SCL and SCA to pins 20 and 21 respectively the bootloader comes up with the message "The bootloader reported error 'Packet data invalid: the packet sent to the bootloader does not conform to the required packet format, see the Bootloader section in the System Reference Guild for the correct format'."
The strange thing is that when I unplug these 2 wires the bootloader works as normal. If I bootload the application and then connect the wires the system does not communicate over the I2C bus. If I can get this to work it will make such a huge difference to my project. Does anyone know what is going on here? Can I just not use these pins?