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CM8870 Datasheet(PDF) 5 Page - California Micro Devices Corp

Part No. CM8870
Description  CMOS Integrated DTMF Receiver
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Maker  CALMIRCO [California Micro Devices Corp]
Homepage  http://www.calmicro.com
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CM8870 Datasheet(HTML) 5 Page - California Micro Devices Corp

   
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© 2000 California Micro Devices Corp. All rights reserved.
9/28/2000
5
CM8870/70C
CALIFORNIA MICRO DEVICES
215 Topaz Street, Milpitas, California 95035
Tel: (408) 263-3214
Fax: (408) 263-7846
www.calmicro.com
Functional Description
The CAMD CM8870/70C DTMF Integrated Receiver provides
the design engineer with not only low power consumption, but
high performance in a small 18-pin DIP, SOIC, or 20-pin PLCC
package configuration. The CM8870/70C’s internal architec-
ture consists of a bandsplit filter section which separates the
high and low tones of the received pair, followed by a digital
decode (counting) section which verifies both the frequency
and duration of the received tones before passing the result-
ant 4-bit code to the output bus.
Filter Section
Separation of the low-group and high-group tones is achieved
by applying the dual-tone signal to the inputs of two 9th-order
switched capacitor bandpass filters. The bandwidths of these
filters correspond to the bands enclosing the low-group and
high-group tones (See Figure 3). The filter section also
incorporates notches at 350 Hz and 440 Hz which provides
excellent dial tone rejection. Each filter output is followed by a
single order switched capacitor section which smooths the
signals prior to limiting. Signal limiting is performed by high-
gain comparators. These comparators are provided with a
hysteresis to prevent detection of unwanted low-level signals
and noise. The outputs of the comparators provide full-rail
logic swings at the frequencies of the incoming tones.
Decoder Section
The CM8870/70C decoder uses a digital counting technique
to determine the frequencies of the limited tones and to verify
that these tones correspond to standard DTMF frequencies.
A complex averaging algorithm is used to protect against tone
simulation by extraneous signals (such as voice) while
providing tolerance to small frequency variations. The
averaging algorithm has been developed to ensure an
optimum combination of immunity to “talk-off” and tolerance to
the presence of interfering signals (third tones) and noise.
When the detector recognizes the simultaneous presence of
two valid tones (known as “signal condition”), it raises the
“Early Steering” flag (ESt). Any subsequent loss of signal
condition will cause ESt to fall.
Steering Circuit
Before the registration of a decoded tone pair, the receiver
checks for a valid signal duration (referred to as “character-
recognition-condition”). This check is performed by an
external RC time constant driven by E
St.
A logic high on ESt
causes V
C (See Figure 4) to rise as the capacitor discharges.
Providing signal condition is maintained (ESt remains high) for
the validation period (t
GTP), VC reaches the threshold (VTSt) of
the steering logic to register the tone pair, thus latching its
corresponding 4-bit code (See Figure 2) into the output latch.
At this point, the GT output is activated and drives VC to V
DD.
GT continues to drive high as long as ESt remains high,
signaling that a received tone pair has been registered. The
contents of the output latch are made available on the 4-bit
output bus by raising the three-state control input (TOE) to a
logic high. The steering circuit works in reverse to validate the
interdigit pause between signals. Thus, as well as rejecting
signals too short to be considered valid, the receiver will
tolerate signal interruptions (drop outs) too short to be
considered a valid pause. This capability together with the
capability of selecting the steering time constants externally,
allows the designer to tailor performance to meet a wide
variety of system requirements.
Guard Time Adjustment
In situations which do not require independent selection of
receive and pause, the simple steering circuit of Figure 4 is
applicable. Component values are chosen according to the
following formula:
t
REC = tDP + tGTP
t
GTP » 0.67 RC
The value of t
DP is a parameter of the device and tREC is the
minimum signal duration to be recognized by the receiver. A
value for C of 0.1 uF is recommended for most applications,
leaving R to be selected by the designer. For example, a
suitable value of R for a t
REC of 40 milliseconds would be 300K.
A typical circuit using this steering configuration is shown in
Figure 1. The timing requirements for most telecommunica-
tion applications are satisfied with this circuit. Different
steering arrangements may be used to select independently
the guardtimes for tone-present (t
GTP) and tone absent (tGTA).
This may be necessary to meet system specifications which
place both accept and reject limits on both tone duration and
interdigit pause.
Guard time adjustment also allows the designer to tailor
system parameters such as talk-off and noise immunity.
Increasing t
REC improves talk-off performance, since it reduces
the probability that tones simulated by speech will maintain
signal condition for long enough to be registered. On the
other hand, a relatively short t
REC with a long tDO would be
appropriate for extremely noisy environments where fast
acquisition time and immunity to drop-outs would be require-
ments. Design information for guard time adjustment is shown
in Figure 5.
Input Configuration
The input arrangement of the CM8870/70C provides a
differential input operational amplifier as well as a bias source
(V
REF) which is used to bias the inputs at mid-rail.
Provision is made for connection of a feedback resistor to the
op-amp output (GS) for adjustment of gain.
In a single-ended configuration, the input pins are connected
as shown in Figure 1, with the op-amp connected for unity
gain and VREF biasing the input at ½ V
DD. Figure 6 shows the
differential configuration, which permits the adjustment of gain
with the feedback resistor R5.
Clock Circuit
The internal clock circuit is completed with the addition of a
standard television color burst crystal or ceramic resonator
having a resonant frequency of 3.579545 MHz. The
CM8870C in a PLCC package has a buffered oscillator output
(OSC3) that can be used to drive clock inputs of other devices
such as a microprocessor or other CM887X’s as shown in
Figure 7. Multiple CM8870/70Cs can be connected as shown
in figure 8 such that only one crystal or resonator is required.


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