UM0932 datasheet(17/43 Pages) STMICROELECTRONICS

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UM0932

Host interface

3.2.5

Local port arbitration rules

The ST7580 modem is always the communication master. In case of no local transfer, the

ST7580 can initiate a local communication without taking into account the external host

status. On the other hand, when the external host wants to send a local frame, it must first

send a request through the T_REQ (transmitting request) input port. Then the ST7580

answers with a status message allowing or not the reception of a frame (or any other

command).

3.2.6

Frame delimitation

Data coming from the UART serial port are an asynchronous flow of bytes. In order to divide

the bytes flow into a frames flow, the ST7580 communication protocol uses two methods:

length and timeout.

When the number of bytes received reaches the expected frame length, in accordance with

the frame type's characteristics, the frame reception ends.

The time interval between two consecutive characters (two local frames including start and

stop bits) in a local frame must not exceed TIC (timeout inter-character): the receiving part

(ST7580 host interface or external host) no longer accepts any character after this delay

expiration.

Figure 9. Timeout inter character TIC

The timeout inter character (TIC) is set by default at 10 ms after a reset and it can be

modified by writing a dedicated MIB object (host interface timeout 09h, Section 4.2.9).

If the length and the checksum are both correct, the received frame is accepted, otherwise

all previous characters are discarded.

3.2.7

Data communication from the ST7580 to the external host

When the ST7580 needs to transmit a frame to the host, it can directly send it without any

previous request. The local frame is automatically built with an STX value equal to 02h.

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

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

If one of the following cases is verified on the RXD line:



Negative acknowledgement (NAK)



Any other frame



No frame within TACK interval.

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