BU8871F Datasheet, PDF (9/12 Pages) Rohm – DTMF receiver for telephones

Communication ICs
(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 below.
The relation between a momentary falter in the ESt guard
time (t.GL), a momentary HIGH level in the ESt guard time
(tGH), and the time constant is shown below. Figure 10
shows a timing diagram for guard times.
BU8871F
FOperation notes
S Oscillation
Oscillation frequency precision can be a problem with ce-
ramic resonators. Before including a ceramic resonator
in your design, please consult the resonator manufactur-
er to make sure this will not be a problem.
Also, if an external clock is being injected, a DC blocking
capacitor must be inserted. Select a capacitor that will
neither attenuate the frequency components or put an
excessive load on the drive side.
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