BU8874 Datasheet, PDF (5/10 Pages) Rohm – DTMF receiver for telephones

Communication ICs
FCircuit 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) frequen-
cies. The separated tonesre converted into square
waves and fed to a DIGITAL DETECTOR. The DIGITAL
DETECTOR checks the two tones to see if they are with-
in 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 de-
tected 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 4 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 PAR-
ALLEL!SERIAL 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 5 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 de-
coded properly. Any ACK pulses in excess of four are ig-
nored until ESt goes HIGH again. Table 1 shows the for-
mat of serial data output from the SD pin.
BU8874 / BU8874F
(2) Power down interface
The power down mode is activated by applying a rising
edge at the PWDN pin when the ACK pin is LOW. The
ACK pin may be taken from LOW to HIGH and back to
LOW again while the circuit is in the power down mode.
To return to the normal operation mode, set PWDN to
LOW. After returning from the power down mode to the
normal operation mode, if a valid DTMF signal is still be-
ing held (from prior to entering the power down mode),
a second reading of the data can be performed (the first
reading following recovery to the normal operation
mode) by inputting the ACK pulse sequence. Figure 3
shows this status.
If the circuit enters the power down mode n times, data
can be read (n)1) times (while ESt is HIGH), as long as
input of the DTMF signal continues.
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