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MT8888C

Integrated DTMF Transceiver with Intel Micro Interface

Zarlink Semiconductor Inc 

MT8888C

Data Sheet

VDD

St/GT

tGTP = (RPC1) In [VDD / (VDD-VTSt)]

tGTA = (R1C1) In (VDD/VTSt)

RP = (R1R2) / (R1 + R2)

C1

R1

ESt

VDD

St/GT

R2

a) decreasing tGTP; (tGTP < tGTA)

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

tGTA = (RpC1) In (VDD/VTSt)

RP = (R1R2) / (R1 + R2)

C1

R1

ESt

R2

b) decreasing tGTA; (tGTP > tGTA)

Figure 6 - Guard Time Adjustment

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 9.

6.0 Call Progress Filter

A call progress mode, using the MT8888C, can be selected allowing the detection of various tones, which identify

the progress of a telephone call on the network. The call progress tone input and DTMF input are common,

however, call progress tones can only be detected when CP mode has been selected. DTMF signals cannot be

detected if CP mode has been selected (see Table 7). Figure 8 indicates the useful detect bandwidth of the call

progress filter. Frequencies presented to the input, which are within the ‘accept’ bandwidth limits of the filter, are

hard-limited by a high gain comparator with the IRQ/CP pin serving as the output. The squarewave output obtained

from the schmitt trigger can be analyzed by a microprocessor or counter arrangement to determine the nature of

the call progress tone being detected. Frequencies which are in the ‘reject’ area will not be detected and

consequently the IRQ/CP pin will remain low.

7

Zarlink Semiconductor Inc.

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