I built a DTMF decoder a few years back using a CM8880 and I would have been lost without the use of a LCD connected. I first wrote a routine to display the DTMF tone on the LCD, so as I played with the keypad, I could see the keys that I pushed. Once I was convinced my programming worked, I then started writing the second routine for the passcode, and with the LCD attached I could do checks within the programming when I ran into troubles. I hope this helps!

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