UM0934 datasheet(43/113 Pages) STMICROELECTRONICS

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UM0934

Host interface

Doc ID 17389 Rev 2

43/113

Data communication from the external host to the ST7570

When the external host needs to initiate a data transfer to the ST7570, it must set the

T_REQ signal at low level. The ST7570 answers within the TPOLL delay with the status

message confirming or not the data channel availability.

If the communication is possible, the external host can start sending a local frame within the

TSR delay. The T_REQ signal is set to a high logic value as soon as the first character (STX)

of the local frame (Section 5.2.1) has been sent. If the first byte of the local frame is not

received before the TSR delay, the ST7570 ignores it.

At the end of the data reception on the RXD line, the ST7570 sends an acknowledgement

message on the TXD line to inform the status of the transmission (ACK or NAK).

Figure 17.

Data flow from external host to the ST7570

If the length and the checksum of the local frame are both correct, the ST7570

acknowledges with an ACK character. In other cases, it answers with an NAK character.

In the case of an NAK response or no acknowledgement from the ST7570 within the TACK

timeout, a complete sequence must be restarted to repeat the frame.

Data communication from the ST7570 to the external host

When the ST7570 needs to transmit a frame to the host, it can directly send it without any

previous request.

Figure 18.

Data flow from the ST7570 to the external host

If the length and the checksum of the local frame are both correct, the external host micro

acknowledges with an ACK character. In other cases, it answers with an NAK character.

T_REQ

Local Frame from External Host

Status

ACK

RXD

TXD

TPOLL

TSR

TACK

T_REQ

NAK

Local Frame

from ST7570

RXD

TXD

TACK

TWBC

ACK

Local Frame

from ST7570

Local Frame

from ST7570

TACK

TWBC

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