ST75C520 datasheet(10/45 Pages) STMICROELECTRONICS

10 / 45 page

IV - FUNCTIONAL DESCRIPTION

IV.1 - System Architecture

The chip allows the design of a complete FAX

data-pump without any external component. Aver-

satile dual port RAM allows an easy interface with

most micro-controllers.

IV.2 - Operation

IV.2.1 - Modes

The modem implementation is fully compatible with

FAX modulation recommendations. The modula-

tion can be either Trellis Coded Modulation (TCM)

as in V.17 14400, 12000, 9600, 7200bps rates,

Quadrature Amplitude Modulation (QAM)

as in

V.29 9600, 7200, 4800 and V.27ter 4800 and

2400bps. Other modes of operation include tone

and DTMF detection or generation, or speech

mode.

IV.2.2 - Transmitter Description

The signal pulses are shaped in a dedicated filter

further combined with a compromise transmit

equalizer suited for transmission over strongly dis-

torted lines. 3 different compromise equalizers are

available and can be selected by software.

IV.2.3 - Receiver Description

The receiver section handles complex signals and

uses a fractionally spaced complex equalizer. It is

able to cope with distant modem timing drifts up to

-4 as specified in the ITU-T recommendations. It

also compensate for frequency drift up to 10Hz and

for phase jitter at multiple and simultaneous fre-

quencies.

IV.2.4 - Tone Generator Description

Four tones can be simultaneously generated by the

ST75C520. The tones are determined by their

frequenciesand by the output amplitude level. A set

of specific commands are also available for DTMF

generation (using two of the four generators avail-

able).

IV.2.5 - Tone Detector Description

Sixteen tones can be simultaneously detected by

the ST75C520. Each of the tones to be detected is

defined by the coefficients of a 4th order program-

mable IIR. Detection thresholds are also program-

mable from -45dBm up to -10dBm. DTMF detection

is also available and is performed by a specific filter

section (that requires no programming).

IV.2.6 - DTMF Detector Description

A DTMF Detector is included in the ST75C520, it

allows detection of valid DTMF Digits. Avalid DTMF

Digit is defined as a dual Tone with a total power

higher than -35dBm, a duration higher than 40ms

and a differential amplitude within 8dB (negative or

positive).

IV.2.7 - Voice Mode

The ST75C520 voice mode allows the implemen-

tation of enhanced telephone functions like an-

swering machines. The incoming samples

(9600Hz), received from the line are PCM A-law

coded and writen into the dual port RAM. The

outpoing samples are decompressed using the

same A-law and output to the telephone line.

The voice mode is entered using a CONF com-

mand, it can be either transmit voice from the dual

RAM Tx buffer to the telephone line, receive voice

from the telephone line to the dual RAM buffer, or

both of these functions simultaneously. The format

of the signal is A-law coded without complementa-

tion of the even bits. The buffer mechanism, be-

tween the host micro-controler and the ST75C520,

is identical to the mechanism used for parallel data

exchanges except that it starts immediately after

CONF command, the size of the transmit and

received buffer, are and must be 8 bytes, there is

no need for a XMIT command, and if an overrun or

underrun condition occurs no error will be reported

to the host processor.

IV.2.8 - Analog Loop Back Test Mode

In any transmission standard and serial data for-

mat, the ST75C520 can be configured for analog

loop back test.

IV.2.9 - Low Power Mode

Sleep state can be attained by a SLEEP command.

Activating the reset signal will wake up the data-

pump. When in sleep mode, the dual port RAM is

unavailable and the clocks are disabled.

When entering the low power mode, the ST75C520

stops its oscillator, all the peripherals of the DSP

core are stopped in order to reduce the power

consumption. The dual RAM is made inacessible.

The ST75C520 can be awakened by a hardware

reset.

There is a maximum time of 20ms to restart the

oscillator after waking up and an additional 5ms

after the interrupt to be able to accept any com-

mand coming from the host.

ST75C520

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