Datasheet BU8871F Datasheet (ROHM)

Communication ICs

DTMF receiver for telephones

BU8871F

The BU8871F is a DTMF receiver ICs developed for use in telephone answering machines, and converts 16 different
types of DTMF signals into 4-bit binary serial data. It features a wide dynamic range, eliminating the need for an external
input amplifier. Expertise from a number of companies has been incorporated into these products to enable guard time
control through a host microcomputer.

Applications
Telephone answering machines

Features

1) Dynamic range of 45dB. (internal AGC)

2) 4-bit binary serial data output.

3) Guard time can be controlled through host microcon­troller.

Block diagram

4) Input pins equipped with hysteresis. (ACK pin)

5) 4.19MHz crystal resonator can be used.

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Communication ICs BU8871F

FAbsolute maximum ratings (Ta=25_C)

FRecommended operating conditions (Ta=25_C)

FPin descriptions

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Input / output circuits

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FElectrical characteristics
DC characteristics (unless otherwise noted, Ta=25_C, V

DD=5.0V)

AC characteristics (unless otherwise noted, Ta=25_C, V

DD=5.0V)

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Measurement circuits

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Communication ICs BU8871F

Circuit operation
(1) An overview of operation
A DTMF signal is supplied to the INPUT pin and applied
to a pair of 6th-order bandpass filters, which separate the
DTMF signal into its high (COL) and low (ROW) frequen­cies. The separated tones are converted into square
waves and fed to a digital detector. (See the block dia­gram.)
The digital detector checks the two tones to see if they
are within the valid DTMF frequency bands. If they are,
it sends a DETECT signal to the steering circuit, and
sends the appropriate column and row address signals
to a code converter.
The code converter encodes the received and detected
DTMF signal, and outputs an ENABLE signal to the
steering circuit.
Based on the DETECT and ENABLE signals, the steer­ing circuit outputs an Early Steering (ESt) signal, which
sets the ESt pin to HIGH, indicating that a valid DTMF
signal has been detected.
If a series of pulses is input at the ACK pin while ESt is
HIGH, a decoded DTMF signal is output to the SD pin as
a binary code. (See Figure 3 for the overall timing.)
If a pulse sequence is input at the ACK pin, the data is
latched at the rising edge of the first pulse by a parallel­serial converter, and at the same time, the LSB is output
from the SD pin. Following this, three bits of data are out­put from the SD pin for each bit of each pulse in the pulse
sequence input from the ACK pin. As a result, a total of
four bits of data are output for the four pulses. (See Fig­ure 4 for the ACK and SD timing.)
If the pulse sequence input to the ACK pin consists of
three or fewer pulses, the next DTMF input cannot be de­coded properly . Any ACK pulses in excess of four are ig­nored until ESt goes HIGH again.
Table 1 shows the format of serial data output from the
SD pin.

(2) Overall timing chart

(3) ACK and SD timing

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Communication ICs BU8871F

Circuit operation
(4) Serial data correspondence table

Application example

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Communication ICs BU8871F

Selecting attached components
(1) Power supply components

C502 : This is the V

100µF . The maximum recommended operating
power supply voltage is 5.25V, so a voltage
withstand value of 6.3V is sufficient.

JP501 : This is normally shorted. T o test the current con-

sumption of the IC, insert a DC ammeter in place
of JP501.

(2) Oscillation components

X501 : Use a crystal or ceramic resonator with an os-

cillation frequency of 4.194304MHz. If using a
ceramic resonator, there may be problems with
the precision of the oscillation frequency, so
please consult the manufacturer of the resona­tor to make sure problems will not occur.

C591 : If you are using the X501 dedicated resonator

designed for DTMF receivers, capacitor C591
should be left open.
If you are injecting an external clock, X501
should be omitted and DC blocking capacitor
C591 used in its place. Typically, this capacitor
should be 47nF.

(3) DTMF input

DD bypass capacitor, and is normally

C501 : This is the DC blocking capacitor. Select a ca-

pacitor that will pass DTMF signals (greater
than 697Hz) without significantly attenuating
the signals.

JP592 : If DTMF signals are being input directly, both

ends should be shorted.
Q591
R591  R595
C592, C593

Use these to increase the sensitivity of
the DTMF receiver.

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Communication ICs BU8871F

(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

GH), and the time constant is shown below. Figure 10

(t
shows a timing diagram for guard times.

FOperation notes
S Oscillation
Oscillation frequency precision can be a problem with ce­ramic resonators. Before including a ceramic resonator
in your design, please consult the resonator manufactur­er to make sure this will not be a problem.

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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.

Communication ICs BU8871F

This LSI is not equipped with the power-on reset function. Also, since the internal circuit (flip-flop circuit) becomes unsta­ble at the rising edge of the power supply , the internal circuit is initialized as shown below by the first DTMF sequence
received after the rising edge of the power supply . Therefore, input four dummy ACK pulses before the DTMF reception.

Electrical characteristic curves

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Application board patterns

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External dimensions (Units: mm)

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