M-8870-01 View Datasheet(PDF) - Unspecified

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M-8870-01

DTMF Receiver

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M-8870

Functional Description

M-8870 operating functions (see Figure 2) include a bandsplit

filter that separates the high and low tones of the received pair,

and a digital decoder that verifies both the frequency and dura-

tion of the received tones before passing the resulting 4-bit code

to the output bus.

Filter

The low and high group tones are separated by applying the

dual-tone signal to the inputs of two 6th order switched capacitor

bandpass filters with bandwidths that correspond to the bands

enclosing the low and high group tones. The filter also incorpo-

rates notches at 350 and 440 Hz, providing excellent dial tone

rejection. Each filter output is followed by a single-order

switched capacitor section that smooths the signals prior to lim-

iting. Signal limiting is performed by high-gain comparators pro-

vided with hysteresis to prevent detection of unwanted low-level

signals and noise. The comparator outputs provide full-rail logic

swings at the frequencies of the incoming tones.

Decoder

The M-8870 decoder uses a digital counting technique to deter-

mine the frequencies of the limited tones and to verify that they

correspond to standard DTMF frequencies. A complex averag-

ing algorithm is used to protect against tone simulation by extra-

neous 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 simulta-

neous 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 a decoded tone pair is registered, the receiver checks for

a valid signal duration (referred to as “charac-

ter-recognition-condition”). This check is performed by an exter-

nal RC time constant driven by ESt. A logic high on ESt causes

VC (see Figure 2) to rise as the capacitor discharges. Provided

that signal condition is maintained (ESt remains high) for the val-

idation period (tGTF), VC reaches the threshold (VTSt) of the

steering logic to register the tone pair, thus latching its corre-

sponding 4-bit code (see Table 3) into the output latch. At this

point, the GT output is activated and drives VC to VDD. GT con-

tinues to drive high as long as ESt remains high. Finally, after a

Figure 3 Basic Steering Circuit

short delay to allow the output latch to settle, the “delayed steer-

ing” output flag (StD) goes 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 (OE) to a logic high. The steering circuit works in re-

verse to validate the interdigit pause between signals. Thus, as

well as rejecting signals too short to be considered valid, the re-

ceiver will tolerate signal interruptions (dropouts) too short to be

considered a valid pause. This capability, together with the abil-

ity to select the steering time constants externally, allows the de-

signer to tailor performance to meet a wide variety of system

requirements.

Figure 4 Single-Ended Input Configuration

Guard Time Adjustment

Where independent selection of signal duration and interdigit

pause are not required, the simple steering circuit of Figure 3 is

applicable. Component values are chosen according to the for-

mula:

tREC = tDP + tGTP

tGTP ≅ 0.67 RC

The value of tDP is a parameter of the device and tREC is the mini-

mum signal duration to be recognized by the receiver. A value

for C of 0.1 µF 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 ms would be 300 kΩ. A typical circuit using this

steering configuration is shown in Figure 4. The timing require-

ments for most telecommunication applications are satisfied

with this circuit. Different steering arrangements may be used to

select independently the guard times for tone-present (tGTP) and

tone-absent (tGTA). This may be necessary to meet system

specifications that place both accept and reject limits on both

tone duration and interdigit pause.

40-406-00011, Rev. F

Page 2

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