Whilst I like your idea, of using interrupts, I think I'll leave it for this iteration. I'm combining both my frequency counter and a CAN Bus module (MCP2515) onto the RPi SPI Interface. The MCP2515 will be connected to the RPi interrupt line so I think I'll make it simple and have the frequency polled. Perhaps you can have two devices on the one RPi interrupt line, I'd have to get into that and do some research. For the moment I'll just poll it.