ROHM BU8871F

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
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4) Input pins equipped with hysteresis. (ACK pin)
5) 4.19MHz crystal resonator can be used.
Communication ICs
BU8871F
FAbsolute maximum ratings (Ta=25_C)
FRecommended operating conditions (Ta=25_C)
FPin descriptions
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Communication ICs
Input / output circuits
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BU8871F
Communication ICs
BU8871F
FElectrical characteristics
DC characteristics (unless otherwise noted, Ta=25_C, VDD=5.0V)
AC characteristics (unless otherwise noted, Ta=25_C, VDD=5.0V)
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Communication ICs
Measurement circuits
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BU8871F
Communication ICs
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.
BU8871F
(2) Overall timing chart
(3) ACK and SD timing
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Communication ICs
Circuit operation
(4) Serial data correspondence table
Application example
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BU8871F
Communication ICs
BU8871F
Selecting attached components
(1) Power supply components
C502 : This is the VDD bypass capacitor, and is normally
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. To test the current consumption of the IC, insert a DC ammeter in place
of JP501.
(2) Oscillation components
X501 : Use a crystal or ceramic resonator with an oscillation 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
C501 : This is the DC blocking capacitor. Select a capacitor 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
Use these to increase the sensitivity of
R591 R595
the DTMF receiver.
C592, C593
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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
(tGH), and the time constant is shown below. Figure 10
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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Communication ICs
Application board patterns
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BU8871F
Communication ICs
BU8871F
External dimensions (Units: mm)
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