MT8889CS datasheet(5/18 Pages) MITEL

5 / 18 page

MT8889C/MT8889C-1

4-111

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.

Call Progress Filter

call

progress

mode,

using

the

MT8889C/

MT8889C-1, 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

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.

Figure 8 - Call Progress Response

LEVEL

(dBm)

FREQUENCY (Hz)

-25

250

500

750

= Reject

= May Accept

= Accept

Figure 7 - Receiver Timing Diagram

Vin

ESt

St/GT

RX0-RX3

Read

Status

IRQ/CP

EVENTS

tREC

tID

tDO

TONE #n

TONE

#n + 1

TONE

#n + 1

tDP

tDA

tGTP

tGTA

tPStRX

tPStb3

DECODED TONE # (n-1)

# n

# (n + 1)

VTSt

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