MT8880CSSLASHCS-1 datasheet(6/18 Pages) MITEL

6 / 18 page

MT8880C/MT8880C-1 ISO2-CMOS

4-38

Figure 9 - 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

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

the segment length as described above the tone

output signal frequency will 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 10

that the distortion products are very low in amplitude.

Figure 10 - Spectrum Plot

Scaling Information

10 dB/Div

Start Frequency = 0 Hz

Stop Frequency = 3400 Hz

Marker Frequency = 697 Hz and

1209 Hz

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