I have recently built my own version of your excellent Opensprinkler. I have used the circuit diagram for V2.3 using the I2C LCD and the solenoid boost circuit. I am using the ATMEGA644P MCU. I have confused myself as to which version of the firmwate I should be using as looking through the code for the Generation2 option, the LCD in use seems to be the parallel version. Could you please point me in the right direction as to the correct version of the firmware that I should be compiling to work with the board that I have built please? I think I am missing something! Thank you.
Second, if you built your own: version 2.3 uses ATMEGA1284, not ATMEGA644. You can still flash earlier versions of firmware (prior to version 2.1.6) if you have 644, but the latest firmware is too large to fit on 644, so you will need 1284. Also, a blank MCU does not come with bootloader, so either you can use a ISP programmer to flash the firmware, or you use ISP programmer to flash the bootloader then you can flash any firmware using the USB port.