ST75C540 datasheet(60/84 Pages) STMICROELECTRONICS

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X - PARALLEL DATA EXCHANGE

HDLC

UART

BUFFERS

MODUL.

DEMOD.

BUFFERS

IT2

IT3

Telephone

Line

Control

Data

HDLC

UART

Figure 23

X.1 - Overview

While transmiting (respectively receiving) data to

(from) the telephone line data are exchanged be-

tween the host and the ST75C530/540.

Two totaly independent channels are provived for

transmit and receive data. Even while using half

duplex modes of operation, the transmitted data

comes from the transmit buffers and the receive

data arrives in the receive buffers.

Two independent interrupts, IT2 (for transmit) and

IT3 (for receive) are available for synchronizing the

ST75C530/540 and the host. An additional IT0

interrupt will signal the errors in the synchronization

mechanism.

The equivalent data flow is as follows (see Fig-

ure 20).

The ST75C530/540 has a buit-in HDLC capability.

This feature automatically performs HDLC fram-

ing/deframing, CRC generation/detection and “0”

insertion/deletion. The ST75C530/540 have also

UART capability, the format of data is selected by

the FORM command described bellow.

X.2 - Transmit Buffers

Two identical buffers are provided to exchange the

dat a bet ween th e host in terface an d the

ST75C530/540. When the host is writing data into

a buffer, the ST75C530/540 is transmitting the

other one. After that, both the host and the

ST75C530/540switch to use the other buffer. This

mechanism, called “Double-Buffering”, ensures

that the host has the maximum time to fill one

buffer.

The DUAL Ram area associated with the transmit

buffers is as following table.

Name

Address

Description

DTTBS0

$2E

Buffer 0 Status Byte

DTTBS0 [0]

$2F

Buffer 0 Data Byte 0

DTTBS0 [1]

$30

Buffer 0 Data Byte 1

DTTBS0 [2]

$31

Buffer 0 Data Byte 2

DTTBS0 [3]

$32

Buffer 0 Data Byte 3

DTTBS0 [4]

$33

Buffer 0 Data Byte 4

DTTBS0 [5]

$34

Buffer 0 Data Byte 5

DTTBS0 [6]

$35

Buffer 0 Data Byte 6

DTTBS0 [7]

$36

Buffer 0 Data Byte 7

DTTBS1

$37

Buffer 1 Status Byte

DTTBS1 [0]

$38

Buffer 1 Data Byte 0

DTTBS1 [1]

$39

Buffer 1 Data Byte 1

DTTBS1 [2]

$3A

Buffer 1 Data Byte 2

DTTBS1 [3]

$3B

Buffer 1 Data Byte 3

DTTBS1 [4]

$3C

Buffer 1 Data Byte 4

DTTBS1 [5]

$3D

Buffer 1 Data Byte 5

DTTBS1 [6]

$3E

Buffer 1 Data Byte 6

DTTBS1 [7]

$3F

Buffer 1 Data Byte 7

Bit 0 (LSB) of the Buffer 0 Data Byte 0 is the first in

time to be transmited.

According to the Data Format, the Status byte of a

buffer has different meanings. However a value of

0 signals to the host that a buffer is empty. This

value is set by the ST75C530/540 each time it has

emptied the buffer. After having used one buffer,

the host must select the other buffer for the next

operation. The host must start with the Buffer 0 as

soon as the

ST_106 signal goes on and BEFORE

the XMIT 1 command is sent.

A mechanism of interruption (

IT2 for Transmit) is

associated with the data buffer managment. Each

time a buffer is emptied by the ST75C530/540 it

generates an interrupt.

ST75C530 - ST75C540

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