UM0934 datasheet(37/113 Pages) STMICROELECTRONICS

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UM0934

Host interface

Doc ID 17389 Rev 2

37/113

The UART interface has two data channels:

●

TXD carries data from the ST7570 to the host

●

RXD carries data from the host to the ST7570.

The UART communication has the following settings:

●

Half duplex mode

●

Standard NRZ bit coding

●

LSBit first transmission: the bits within each byte are sent LSBit to MSBit

●

The exchanged frames are composed of characters

●

A single character is composed of 1 start bit, 8 data bits, and 1 stop bit

●

0-5 V or 0-3.3 V levels on TXD, RXD, T_REQ signals.

Figure 14

shows the character format:

Figure 14.

Local port character format

5.2

Communication protocol

The host interface process implemented in the ST7570 device performs the following tasks:

●

Frame format definition

●

Reception mechanism: the UART standard on the half duplex data channel is

implemented and collisions are avoided

●

Acknowledgement to received frames

●

Timeout management

●

Error checking: length, syntax, and checksum field of a received frame are controlled

and a repetition is requested in case of error.

5.2.1

Frame types

A frame is a sequence of one or more characters encapsulating data exchanged between

the ST7570 and the external host. The communication protocol defines several frame

formats for the different steps of the communication protocol (access to ST7570 services,

acknowledgement, device status).

All the ST7570 resources and services (PHY layer, MAC layer, and MIB management) are

available through local commands and they can be handled by the external host using the

local communication serial interface and the commands defined in the next paragraphs.

LSB

MSB

Start

Bit

Stop

Bit

IDLE state

tbit

8 data bits

1 character

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