MT8880CSSLASHCS-1 datasheet(5/18 Pages) MITEL

5 / 18 page

ISO2-CMOS

MT8880C/MT8880C-1

4-37

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 9 with a description of the events in Figure 11.

Call Progress Filter

A call progress mode, using the MT8880C/C-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 5). 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

microprocessor

counter

arrangement

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.

DTMF Generator

The DTMF transmitter employed in the MT8880C/C-

1 is capable of generating all sixteen standard DTMF

tone pairs with low distortion and high accuracy. All

frequencies are derived from an external 3.579545

MHz crystal. The sinusoidal waveforms for the

individual tones are digitally synthesized using row

and column programmable dividers and switched

capacitor D/A converters. The row and column tones

are mixed and filtered providing a DTMF signal with

low total harmonic distortion and high accuracy. To

specify a DTMF signal, data conforming to the

encoding format shown in Figure 7 must be written to

the transmit Data Register. Note that this is the same

as the receiver output code. The individual tones

which are generated (fLOW and fHIGH) are referred to

as Low Group and High Group tones. As seen from

the table, the low group frequencies are 697, 770,

852 and 941 Hz. The high group frequencies are

1209, 1336, 1477 and 1633 Hz. Typically, the high

group to low group amplitude ratio (pre-emphasis) is

2dB to compensate for high group attenuation on

long loops.

0= LOGIC LOW, 1= LOGIC HIGH

Figure 7 - Functional Encode/Decode Table

Figure 8 - Call Progress Response

The period of each tone consists of 32 equal time

segments. The period of a tone is controlled by

varying the length of these time segments. During

write operations to the Transmit Data Register the 4

bit data on the bus is latched and converted to 2 of 8

coding for use by the programmable divider circuitry.

This code is used to specify a time segment length

which will ultimately determine the frequency of the

tone. When the divider reaches the appropriate

count, as determined by the input code, a reset pulse

is issued and the counter starts again. The number

FLOW

FHIGH

DIGIT

697

1209

0001

697

1336

0010

697

1477

0011

770

1209

0100

770

1336

0101

770

1477

0110

852

1209

0111

852

1336

1000

852

1477

1001

941

1336

1010

941

1209

1011

941

1477

1100

697

1633

1101

770

1633

1110

852

1633

1111

941

1633

0000

LEVEL

(dBm)

FREQUENCY (Hz)

-25

250

500

750

= Reject

= May Accept

= Accept

MT8880CS/CS-1 Datasheet(PDF) 5 Page - Mitel Networks Corporation

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