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
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-
tion”), it raises the Early Steering flag (ESt). Any subse-
quent loss of signal condition will cause ESt to fall.
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 VC (see Figure 3) to rise as the
capacitor discharges. Provided that 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 corre-
sponding 4-bit code (see Table 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. 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-
Table 1 Pin Functions
Connections to the front-end differential amplifier
Gain select. Gives access to output of front-end amplifier for connection of feedback resistor.
Reference voltage output (nominally VDD/2). May be used to bias the inputs at mid-rail.
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.
3.579545 MHz crystal connected between these pins completes internal oscillator.
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
(see Table 5.)
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 VTSt
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 VTSt detected at St causes the device to
register the detected tone pair and update the output latch. A voltage less than VTSt frees the device to accept a new
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