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
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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
BANDPASS FILTERS
697
BAND
SPLIT
FILTERS
BS1
ZERO
CROSSING
DETECTORS
AMPLITUDE
DETECTORS
770
PRE-PROCESSOR
ANALOG
60 Hz
REJECT
PREEMP
852
941
TIMING
CIRCUITRY
BS2
1209
1336
DV
DV
XEN
1633
÷8
CLOCK
GENERATOR
DATA STROBE
ATB
1477
XIN
CHIP CLOCKS
D1
1Mz
XOUT
D2
OUTPUT
DECODER
OUTPUT
REGISTER
D4
POWER REGULATOR
VOLTAGE
REF
D8
DATA CLEAR
EN
VP
GND
75T204
5V Low-Power Subscriber
DTMF Receiver
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 Mz 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
1
2
3
4
5
6
7
8
9
0
*
#
A
B
C
D
D8
0
0
0
0
0
0
0
1
1
1
1
1
1
1
1
0
D4
0
0
0
1
1
1
1
0
0
0
0
1
1
1
1
0
D2
0
1
1
0
0
1
1
0
0
1
1
0
0
1
1
0
D1
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
TABLE 1: Output Codes
2
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