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MT88L85AN1

3 V Integrated DTMF Transceiver with Power-Down and Adaptive Micro Interface

Microsemi Corporation 

MT88L85

Data Sheet

receiver will tolerate signal interruptions (drop out) too short to be considered a valid pause. This facility, 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.

VDD

MT88L85

VDD

St/GT

ESt

C1

Vc

R1

tGTA = (R1C1) In (VDD / VTSt)

tGTP = (R1C1) In [VDD / (VDD-VTSt)]

Figure 5 - Basic Steering Circuit

Guard Time Adjustment

The simple steering circuit shown in Figure 5 is adequate for most applications. Component values are chosen

according to the following inequalities (see Figure 7):

tREC ≥ tDPmax + tGTPmax - tDAmin

t REC ≤ tDPmin + t GTPmin - tDAmax

tID ≥ tDAmax + tGTAmax - tDPmin

tDO ≤ tDAmin + tGTAmin - tDPmax

The value of tDP is a device parameter (see AC Electrical Characteristics) and tREC is the minimum signal duration

to be recognized by the receiver. A value for C1 of 0.1 µF is recommended for most applications, leaving R1

to be selected by the designer. Different steering arrangements may be used to select independent tone present

(tGTP) and tone absent (tGTA) guard times. This may be necessary to meet system specifications which place both

accept and reject limits on tone duration and interdigital 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 a valid signal condition long enough to be registered. Alternatively, a relatively short tREC with a long tDO

would be appropriate for extremely noisy environments where fast acquisition time and immunity to tone drop-outs

are required. Design information for guard time adjustment is shown in Figure 6. The receiver timing is shown in

Figure 7 with a description of the events in Figure 8.

6

Zarlink Semiconductor Inc.

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