UM0932 datasheet(18/43 Pages) STMICROELECTRONICS

18 / 43 page

Host interface

UM0932

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DocID17385 Rev 3

The ST7580 device repeats the frame only once after a delay corresponding to TACK,

changing the STX value to 03h. Acknowledgement to re-transmitted frames is considered

positive by default, even if the host controller answers with a negative acknowledgement.

Figure 10. Data flow from ST7580 to the external host

3.2.8

Data communication from external host to the ST7580

When the external host needs to initiate a data transfer to the ST7580, it must set the

T_REQ signal at low level. As soon as the ST7580 is not busy, it answers 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 (built with

STX field equal to 02h) within the TSR delay. The T_REQ signal is set to high logic value as

soon as the STX field of the local frame (see Section 3.2.2) has been sent. If the first byte of

the local frame is not received before the TSR delay, the ST7580 ignores it.

Figure 11. Data flow from external host to the ST7580

At the end of the data reception on the RXD line, the ST7580 sends an acknowledgement

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

a TACK interval.

If the length and the checksum of the local frame are both correct, the ST7580

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

In case of a NAK response or no acknowledgement from the ST7580 within the TACK

time-out, a complete sequence must be restarted to repeat the communication.

UM0932 Datasheet(PDF) 18 Page - STMicroelectronics