UM0934 datasheet(42/113 Pages) STMICROELECTRONICS

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Host interface

UM0934

42/113

Doc ID 17389 Rev 2

Acknowledgement messages

After receiving a local frame on the host interface, both the external host and ST7570 must

send an acknowledgement or not acknowledgement message through either TXD (ST7570)

or RXD (external host).

The two messages have the same format for both ST7570 and the external host and they

are 1 byte long.

Table 13

lists their fixed codes.

5.2.2

Local port arbitration rules

The ST7570 modem is always the communication master. In the case of no local transfer,

the ST7570 can initiate a local communication without taking into account the external host

status. On the other hand, when the external host wants to send data (using a local frame),

it must first send a request through the T_REQ (transmitting request) input port. Then, the

ST7570 answers with a status message allowing or not the reception of a frame (or any

other command).

Frame delimitation

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

the byte flow into a frame flow, the ST7570 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 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

(ST7570 host interface or external host) no longer accepts any character after this delay

expiration.

Figure 16.

Timeout inter-character TIC

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

by writing a dedicated MIB object (Section 6.5.12).

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

all previous characters are discarded.

Table 13.

ACK and NAK message codes

Symbol

Definition

Code

ACK

Acknowledgement

06h

NAK

Not acknowledgement

15h

Character

TIC

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