BU8874 Datasheet, PDF (8/10 Pages) Rohm – DTMF receiver for telephones

Communication ICs
(3) DTMF input
BU8874 / BU8874F
C501 :
This is the DC blocking capacitor. Select a ca-
pacitor that will pass DTMF signals (greater
than 697Hz) without significantly attenuating
the signals.
(4) ESt output
The ESt guard time is determined by the CPU of the host
computer, but to reduce the load on the host computer,
the guard time can be set using an external circuit, as
shown in Figure 10.
JP592 : If DTMF signals are being input directly, both
ends should be shorted.
Q591
R591 X R595 ^{Use these to increase the sensitivity of}
C592, C593 ^{the DTMF receiver.}
The relation between a momentary falter in the ESt guard
time (tGL), a momentary HIGH level in the ESt guard time
(tGH), and the time constant is shown below. Figure 12
shows a timing diagram for guard times.
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