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5.21 Pin out of G8870 DTMF decoder IC

The IC can also produce single tones. These are usually generated
for testing purposes. For instance, to generate a 1336-hertz (Hz)
tone equivalent to that of the C2 pin, ground any two row pins
and the C2 pin. This action will generate a single 1336-Hz signal. 75
The same may be done with the row frequencies. Ground any two
column pins with the particular row frequency pin you want to
generate.
DTMF decoding
DTMF decoding is just a little more complex than encoding. Again
the simplicity results from the use of a single IC chip, in this case
the G8870 (see Fig. 5.21).
The decoding chip has a 4-bit latched output labeled Q1 through
Q4. Q4 is the most significant bit (MSB). The current available from
the outputs of Q1 through Q4 is sufficient to light a low-current
LED. Figure 5.22 is a basic receiving circuit. The output from Q1
through Q4 lights the LED and is a binary number. By looking at
Table 5.3, you can determine the binary output that will be displayed
on the Q1 through Q4 for all DTMF signals. The way the circuit is
wired, the binary “1” will be represented by a lit LED.

Microcontroller
The 4-bit number from the G8870 can be connected directly to input
lines of a microcontroller like the PIC 16F84. The microcontroller
can easily read this binary number. We will get to the PIC micro-
controllers in Chap. 7.
Team LRN Sensors