Have you though about flow control. When the MC is going to be too busy it sends an XOFF. When free to receive data it sends XON. Could also be done with hardware handshake.
You only need to do this for the DTMF generation. A keyboard scan can be interrupted without any problem and the LCD should be ok too.
You only need to do this for the DTMF generation. A keyboard scan can be interrupted without any problem and the LCD should be ok too.
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