ST75C540 datasheet(46/84 Pages) STMICROELECTRONICS

46 / 84 page

VIII.2.3 - Modem General Status

The modem general status word is composed of two bytes STATUS[0] and STATUS[1]. Any bit change can

generate an IT4 interrupt. Using the DSIT command allows the selection of the corresponding bit that will

generate an interrupt each time they will change. The default pattern is $3F for STATUS[0] and $FF for

STATUS[1].

The different bits have the following meaning :

STATUS[0]

Field

Pos.

Meaning when set

STA_109

STA_VAD

In FAX MODEM and TONECID modes STA_109 : CCITT Circuit 109 (Carrier Detect). Indicates

that valid data are received.

In CODER and DECODER modes : VAD: Voice Activity Detected

STA_107

CCITT Circuit 107 (Data Set Ready). Valid only in FAX & DATA MODEM modes.

STA_106

CCITT Circuit 106 (Clear To Send). Indicates that the training sequence has been completed

and that any data in the Transmit Buffer will be transmitted. Valid only in FAX & DATA MODEM

modes.

STA_RING

Ring Detected. A valid ring signal is present at the Ring pin. Valid only in Tones modes. The

precise frequency can be read in the optional status byte STAOP2.

STA_CPT0

In TONE and TONECID modes STA_CPT0: Call progress tone detector #0. Low pass filter

650Hz.

STA_CPT1

In TONE and TONECID modes STA_CPT1: Call progress tone detector #1. High pass filter

600Hz.

STA_CPT10

In TONE and TONECID modes STA_CPT10: Signal in Filter #0 is higher than #1.

STA_109F

In FAX MODEM mode, V.22bis mode* and TONECID mode STA_109F: Fast Carrier Detect.

* ST75C540 only

STATUS[1]

Field

Pos.

Meaning

STA_H

Transmit synchronisation in progress. Valid only in FAX & DATA MODEM modes.

STA_TIM*

Handshake timeout. Valid only in Data Modem mode.

STA_CCITT

CCITT 2100Hz versus 2225Hz answer tone detect. Valid if STA_AT is set. Valid only in Tone

mode.

STA_AT

Answer tone (either 2100Hz or 2225Hz) detected. Valid only in Tone mode.

STA_HR

STA_RTRN*

STA_HR : Receive synchronisation in progress. Valid only in Fax Modem mode.

STA_RTRN : Remote retrain detec, valid only in V.32bis/V.32/V.22bis Data Modem modes.

STA_RENEG*

Remote rate negotiation detected, valid only in V.32bis/V.32/V.22bis Data Modem modes.

STA_FLAG

STA_CLR*

STA_FLAG : V.21 channel 2 flag detect. Valid only in FAX Modem mode and Tone mode.

STA_CLR : Remote clear down detected V.32bis/V.32 Data Modem modes.

STA_DTMF

DTMF digit detect. The digit itself is available in the optional status byte STAOP3.

* ST75C540 only

VIII - STATUS DESCRIPTION (continued)

ST75C530 - ST75C540

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