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 CLOCKSXIN
XOUT
1Mz
V P GND
POWER REGULATOR VOLTAGE
REF
60 Hz
REJECT PREEMP
CLOCK
GENERATOR
÷ 8