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CM88L70PEI Datasheet PDF : 8 Pages
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CALIFORNIA MICRO DEVICES
CM88L70/70C
Functional Description
The CAMD CM88L70/70C 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, TSSOP, or
QSOP package configuration. The CM88L70/70Cs internal
architecture 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 resultant 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 CM88L70/70C 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 ESt. A logic high on ESt causes VC (See Figure 4) to rise as
the capacitor discharges. Providing signal condition is maintained
(ESt remains high) for the validation period (tGTP), 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 VDD.
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:
tREC = tDP + tGTP
tGTP » 0.67 RC
The value of tDP 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 tREC of 40
milliseconds would be 300K. A typical circuit using this steering
configuration is shown in Figure 1. The timing requirements for most
telecommunication applications are satisfied with this circuit. Different
steering arrangements may be used to select independently the
guardtimes for tone-present (tGTP) 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 tREC
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 tREC
with a long tDO would be appropriate for extremely noisy environments
where fast acquisition time and immunity to drop-outs would be
requirements. Design information for guard time adjustment is shown
in Figure 5.
Input Configuration
The input arrangement of the CM88L70/70C provides a differential
input operational amplifier as well as a bias source (VREF) 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 VDD. 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 CM887Xs
as shown in Figure 7. Multiple CM88L70/70Cs can be connected as
shown in figure 8 such that only one crystal or resonator is required.
Power Down and Inhibit Mode
A logic high applied to pin 6 (PD) will power down the device to
minimize the power consumption in a standby mode. It stops the
oscillator and functions of the filters.
Inhibit mode is enabled by a logic high input to the pin 5 (INH). It
inhibits the detection of tones representing characters A, B, C and
D. The output code will remain the same as the previous detected
code (see Figure 2).
© 2000 California Micro Devices Corp. All rights reserved.
6/16/2000 215 Topaz Street, Milpitas, California 95035
Tel: (408) 263-3214
Fax: (408) 263-7846
www.calmicro.com
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