How It Works:
The Keypad Interface is used to provide a binary number for the data, and also a Chip Enable output line.
The 4 data lines from the Keypad Interface go straight into the data inputs of the Holtek 9200B DTMF Generator (pins 6, 7, 8 & 9), and the Inverted Chip Enable output on the Keypad Interface goes into the Chip Enable Input on the Holtek 9200B DTMF Generator (pin1).
When the Chip Enable line is at logic 0, a DTMF tone is generated from the DTMF output (pin 13) with respect to ground. This is an 'Active Low' input so logic 0 will enable the chip.
The DTMF output is then taken to the DTMF input line of the Holtek 9170B DTMF Receiver. When a tone is present, the Data Valid output goes high (pin 15). Also the DTMF tone will be decoded to a 4 bit binary number on the output lines (pins 11, 12, 13 and 14).
If using the 9170B Receiver for remote control, the DV output can be used as an interrupt trigger, then the data can be read.
The 4 data output lines from the receiver then go to the 7 segment decoder and display the corresponding DTMF number.
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