MT8885AP datasheet(5/20 Pages) MITEL

5 / 20 page

Advance Information

MT8885

4-55

signal duration to be recognized by the receiver. A

value for C1 of 0.1 µF is recommended for most

Figure 6 - Guard Time Adjustment

VDD

St/GT

ESt

VDD

St/GT

ESt

tGTA = (R1C1) In (VDD/VTSt)

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

RP = (R1R2) / (R1 + R2)

tGTA = (RpC1) In (VDD/VTSt)

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

RP = (R1R2) / (R1 + R2)

a) decreasing tGTP; (tGTP < tGTA)

b) decreasing tGTA; (tGTP > tGTA)

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

A call progress mode, using the MT8885, 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.

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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