75T204
5V Low-Power Subscriber
DTMF Receiver
April 2000
DESCRIPTION
The 75T204 is a complete Dual-Tone Multifrequency
(DTMF) receiver that detects 16 standard digits. No
front-end pre-filtering is needed. The only external
components required are an inexpensive 3.58-MHz
television "colorburst" crystal for frequency reference
and a bias resistor. An Alternate Time Base (ATB)
is provided to permit operation of up to 10 75T204's
from a single crystal. The 75T204 employs state-of-
the-art "switched-capacitor" filter technology,
resulting in approximately 40 poles of filtering, and
digital circuitry on the same CMOS chip. The analog
input signal is pre-processed by 60-Hz reject and
band split filters and then zero-cross detected to
provide AGC. Eight bandpass filters detect the
individual tones. Digital processing is used to
measure the tone and pause durations
(continued)
FEATURES
Intended for applications with less
requirements than the 75T202/75T203
14-pin plastic DIP or 16-pin SO package for
high system density
NO front-end band-splitting filters required
Single low-tolerance 5-volt supply
Detects all 16 standard DTMF digits.
Uses an inexpensive 3.579545-MHz crystal
Excellent speech immunity
Output in 4-bit hexadecimal code
Three-state outputs for microprocessor
interface
BLOCK DIAGRAM
ANALOG
PRE-PROCESSOR
BAND
SPLIT
FILTERS
BS1
852
770
697
941
1209
1336
1477
1633
ZERO
CROSSING
DETECTORS
BANDPASS FILTERS
AMPLITUDE
DETECTORS
DATA STROBE
DV
TIMING
CIRCUITRY
OUTPUT
DECODER
DATA CLEAR
BS2
OUTPUT
REGISTER
D1
DV
D2
D4
D8
EN
ATB
XEN
CHIP CLOCKS
XIN
XOUT
1Mz
VP
GND
POWER REGULATOR
VOLTAGE
REF
60 Hz
REJECT
PREEMP
CLOCK
GENERATOR
8
75T204
5V Low-Power Subscriber
DTMF Receiver
2
DESCRIPTION
(continued)
and to provide output timing and decoding. The
outputs interface directly to standard CMOS circuitry
and are three-state enabled to facilitate bus-oriented
architectures.
ANALOG IN
This pin accepts the analog input. It is internally
biased so that the input signal may be AC coupled.
The input may be DC coupled as long as it does not
exceed the positive supply. Proper input coupling is
illustrated in Figure 1.
The 75T204 is designed to accept sinusoidal input
wave forms but will operate satisfactorily with any
input that has the correct fundamental frequency
with harmonics less then -20 dB below the
fundamental.
CRYSTAL OSCILLATOR
The 75T204 contains an onboard inverter with
sufficient gain to provide oscillation when connected
to a low-cost television "colorburst" crystal. The
crystal oscillator is enabled by tying XEN high. The
crystal is connected between XIN and XOUT. A
1 M
z
10% resistor is also connected between these
pins. In this mode, ATB is a clock frequency output.
Other 75T204's (or 75T202's) may use the same
frequency reference by tying their ATB pins to the
ATB of a crystal connected device. XIN and XEN of
the auxiliary devices must then be tied high and low
respectively. Ten devices may run off a single
crystal-connected 75T204 (or 75T202) as shown in
Figure 2.
OUTPUTS D1, D2, D4, D8 and EN
Outputs D1, D2, D4, D8 are CMOS push-pull when
enabled (EN high) and open circuited (high
impedance) when disabled by pulling EN low.
These digital outputs provide the hexadecimal code
corresponding to the detected digit. The digital
outputs become valid after a tone pair has been
detected (DV is high) and they are then cleared
when a valid pause is timed. The hexadecimal
codes are described in Table 1.
DV
DV signals a detection by going high after a valid
tone pair is sensed and decoded at the output pins
D1, D2, D4, and D8. DV remains high until a valid
pause occurs.
N/C PINS
These pins have no internal connection and may be
left floating.
Output Code
Digit
D8
D4
D2
D1
1
0
0
0
1
2
0
0
1
0
3
0
0
1
1
4
0
1
0
0
5
0
1
0
1
6
0
1
1
0
7
0
1
1
1
8
1
0
0
0
9
1
0
0
1
0
1
0
1
0
*
1
0
1
1
#
1
1
0
0
A
1
1
0
1
B
1
1
1
0
C
1
1
1
1
D
0
0
0
0
TABLE 1: Output Codes
75T204
5V Low-Power Subscriber
DTMF Receiver
4
DTMF DIALING MATRIX
See Figure 3. Please note that column 3 is for special applications and is not normally used in telephone dialing.
FIGURE 3: DTMF Dialing Matrix
FIGURE 4: Timing Diagram
Col 0
Col 1
Col 2
Col 3
Row 1
Row 2
Row 3
1
4
7
*
2
5
8
0
3
6
9
#
A
B
C
D
Row 0
Tone burst 2
Pause
ton
tD
tR
tS
tH
Analog
Input
D1, D2,
D4, D8
DV
Tone burst 1
toff
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