MT88L89 datasheet(7/20 Pages) MITEL

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Advance Information

MT88L89

4-131

Figure 10 - Spectrum Plot

Scaling Information

10 dB/Div

Start Frequency = 0 Hz

Stop Frequency = 3400 Hz

Marker Frequency = 697 Hz and

1209 Hz

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

of time segments is fixed at 32, however, by varying

the

segment

length

described

above

the

frequency can also be varied. The divider output

clocks

another

counter,

which

addresses

the

sinewave lookup ROM.

The lookup table contains codes which are used by

the switched capacitor D/A converter to obtain

discrete and highly accurate DC voltage levels. Two

identical circuits are employed to produce row

and

column tones, which are then mixed using a low

noise summing amplifier. The oscillator described

needs

“start-up”

time

other

DTMF

generators since the crystal oscillator is running

continuously thus providing a high degree of tone

burst

accuracy.

bandwidth

limiting

filter

incorporated and serves to attenuate distortion

products above 8 kHz. It can be seen from Figure 6

that the distortion products are very low in amplitude.

Burst Mode

In certain telephony applications it is required that

DTMF signals being generated are of a specific

duration

determined

either

the

particular

application or by any one of the exchange transmitter

specifications currently existing. Standard DTMF

signal timing can be accomplished by making use of

the Burst Mode. The transmitter is capable of issuing

symmetric bursts/pauses of predetermined duration.

This burst/pause duration is 51 ms±1 ms which is a

standard interval for autodialer and central office

applications. After the burst/pause has been issued,

the appropriate bit is set in the Status Register

indicating that the transmitter is ready for more data.

The timing described above is available when DTMF

mode has been selected. However, when CP mode

(Call Progress mode) is selected, the burst/pause

duration is doubled to 102 ms ±2 ms. Note that when

CP mode and Burst mode have been selected,

DTMF tones may be transmitted only and

not

received. In applications where a non-standard

burst/pause time is desirable, a software timing loop

or external timer can be used to provide the timing

pulses when the burst mode is disabled by enabling

and disabling the transmitter.

Single Tone Generation

A single tone mode is available whereby individual

tones from the low group or high group can be

generated. This mode can be used for DTMF test

equipment

applications,

acknowledgment

tone

generation and distortion measurements. Refer to

Control Register B description for details.

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