ST7538Q datasheet(20/44 Pages) STMICROELECTRONICS

20 / 44 page

Functional description

ST7538Q

20/44

5.5.1

Communication between host and ST7538Q

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” ST7538Q receives data from mains, if RxTx = ”0” ST7538Q transmits data over

the mains).

Communication between Host and ST7538Q 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), ST7538Q 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), ST7538Q enters in an

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

transmitting data present on TxD line.

●

Synchronous mode

In Synchronous Mode ST7538Q is always the master of the communication and

provides the clock reference on CLR/T line.

When ST7538Q is in receiving mode an internal PLL recovers the clock reference.

Data on RxD line are stable on CLR/T rising Edge.

When ST7538Q is in transmitting mode the clock reference is internally generated and

TxD line is sampled on CLR/T rising Edge.

If RxTx line is set to “1” & REG_DATA=”0” (Data Reception), ST7538Q enters in an Idle

State and CLR/T line is forced Low. After Tcc time the modem starts providing received

data on RxD line.

If RxTx line is set to “0” & REG_DATA=”0” (Data Transmission), ST7538Q enters in an

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

transmitting data present on TxD line (Figure 5) .

Figure 4.

Receiving and transmitting data/recovered clock timing

Transmitting Bit Synchronization

CLR/T

RxD

CLR/T

TxD

Receiving Bit Synchronization

TSTH

D03IN1416

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