Dialing Cheatcodes can be a pain in GTA5. Therefore I build a small keyboard emulator which ist entering the cheats for you. The nice thing about the chip I used is that the whole devkit is 10$ and even comes with CapSense etc, which means that instead of buttons for each cheat you can hook up a toy-car or a banana to the chip to activate a cheat.
what ten dollar dev kit are you using STM Discovery? The NXP one ?