M-88L70 CLARE [Clare, Inc.], M-88L70 Datasheet - Page 2

• Manufacturer Part Number: M-88L70
• Description: 3V DTMF Receiver
• Manufacturer: CLARE [Clare, Inc.]

M-88L70
Table 1 Pin Functions
2
Filter
The low and high group tones are separated by applying
the dual-tone signal to the inputs of two 9th order
switched capacitor bandpass filters with bandwidths that
correspond to the bands enclosing the low and high
group tones. The filter also incorporates notches at 350
and 440 Hz, providing excellent dial tone rejection. Each
filter output is followed by a single-order switched capac-
itor section that smoothes the signals prior to limiting.
Signal limiting is performed by high-gain comparators
provided with hysteresis to prevent detection of unwant-
ed low-level signals and noise. The comparator outputs
provide full-rail logic swings at the frequencies of the
incoming tones.
Decoder
The M-88L70 decoder uses a digital counting technique
to determine the frequencies of the limited tones and to
verify that they correspond to standard DTMF frequen-
cies. A complex averaging algorithm is used to protect
against tone simulation by extraneous signals (such as
voice) while tolerating small frequency variations. The
algorithm ensures an optimum combination of immunity
to talkoff and tolerance to interfering signals (third tones)
and noise. When the detector recognizes the simultane-
ous presence of two valid tones (known as “signal condi-
Pin
1
2
3
4
5
6
7
8
9
10
11-14
15
16
17
18
Name
Q1, Q2,
Q3, Q4
OSC1
OSC2
St/GT
V
INH
IN+
StD
V
V
ESt
GS
PD
OE
IN
REF
DD
SS
Non-inverting input
-Inverting input
Gain select. Gives access to output of front-end amplifier for connection of feedback resistor.
Reference voltage output (nominally V
Inhibits detection of tones representing keys A, B, C, and D. This input is internally pulled down.
Power down. Logic high powers down the device and inhibits the oscillator. This input is internally pulled down.
Clock input
Clock output
Negative power supply (normally connected to 0 V).
Tri-state output enable (input). Logic high enables the outputs Q1 - Q4. Internal pullup.
Tri-state outputs. When enabled by OE, provides the code corresponding to the last valid tone pair received
Delayed steering output. Presents a logic high when a received tone pair has been registered and the output latch is
updated. Returns to logic low when the voltage on St/GT falls below V
Early steering output. Presents a logic high immediately when the digital algorithm detects a recognizable tone pair
(signal condition). Any momentary loss of signal condition will cause ESt to return to a logic low.
Steering input/guard time output (bidirectional). A voltage greater than V
register the detected tone pair and update the output latch. A voltage less than V
tone pair. The GT output acts to reset the external steering time constant, and its state is a function of ESt and the
voltage on St. (See Figure 5).
Positive power supply
(see Table 5.)
Connections to the front-end differential amplifier
3.579545 MHz crystal connected between these pins completes internal oscillator.
DD
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/2). May be used to bias the inputs at mid-rail.
Description
tion”), it raises the Early Steering flag (ESt). Any subse-
quent loss of signal condition will cause ESt to fall.
Steering Circuit
Before a decoded tone pair is registered, the receiver
checks for a valid signal duration (referred to as “char-
acter-recognition-condition”). This check is performed
by an external RC time constant driven by ESt. A logic
high on ESt causes V
capacitor discharges. Provided that signal condition is
maintained (ESt remains high) for the validation period
(t
logic to register the tone pair, thus latching its corre-
sponding 4-bit code (see Table 2) into the output latch.
At this point, the GT output is activated and drives V
V
high. Finally, after a short delay to allow the output latch
to settle, the “delayed steering” output flag (StD) goes
high, signaling that a received tone pair has been reg-
istered. The contents of the output latch are made
available on the 4-bit output bus by raising the three-
state control input (OE) to a logic high. The steering cir-
cuit 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 (dropouts) too short to be consid-
GTP
DD
. GT continues to drive high as long as ESt remains
), VC reaches the threshold (V
TSt
TSt
detected at St causes the device to
C
TSt
(see Figure 3) to rise as the
frees the device to accept a new
TSt
) of the steering
C
Rev. 1
to