ST7540 datasheet(20/44 Pages) STMICROELECTRONICS

20 / 44 page

Functional description

ST7540

20/44

Figure 6.

Synchronous and Asynchronous ST7540/Host Controller interfaces

ST7540 allows to interface the Host Controller using a five line interface (RxD,TxD,RxTx,

CLR/T, & REG_DATA) in case of Synchronous mains access or using a 3 line interface

(RxD,TxD & RxTx) in Asynchronous mains access. Since Control Register is not accessible

in Asynchronous mode, in this case REG_DATA pin must be tied to GND.

6.5.1

Communication between Host and ST7540

The Host can achieve the Mains access by selecting REG_DATA=”0” and the choice

between Data Transmission or Data Reception is performed by selecting RxTx line (if RxTx

=“1” ST7540 receives data from mains, if RxTx=”0” ST7540 transmits data over the mains).

Communication between Host and ST7540 is different in Asynchronous and Synchronous

mode:

●

Asynchronous mode:

In Asynchronous Mode, data are exchanged without any data Clock reference. The

host controller has to recover the clock reference in receiving Mode and control the Bit

time in transmission mode.

If RxTx line is set to “1” & REG_DATA=”0” (Data Reception), ST7540 enters in an Idle

State. After Tcc time the modem starts providing received data on RxD line.

If RxTx line is set to “0” & REG_DATA=”0” (Data Transmission), ST7540 enters in an

Idle State and transmission circuitry is switched on. After Tcc time the modem starts

transmitting data present on TxD line.

RxD

CLR/T

REG_DATA

RxTx

ST7540

Host Controller

TxD

UART/Asynchronous

Data Interface

RxD

CLR/T

REG_DATA

RxTx

ST7540

Host Controller

TxD

SPI/Synchronous

Data Interface

D03IN1415

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