ST7580

Data link layer

Doc ID 022644 Rev 1

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10

Data link layer

The data link layer implemented in the ST7580 provides some basic services:

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Encapsulation of user payloads into frames and frame delimitation

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Error detection and cancelling of corrupted frames

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Sniffer functionality for corrupted frames

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Encryption and authentication based on AES 128-bit algorithm

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Traffic statistics.

10.1

Data link frame

Figure 15

shows the data link frame structure:

Figure 15.

Data link frame structure (length in bytes)

The meaning of each field is as follows:

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Length: length in bytes of the payload and CRC fields

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Payload: information bytes

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CRC: CRC check. Its length, endianness and the fields involved in the calculation

(length and payload, or payload only) can be chosen by the external host.

10.2

Error detection and sniffer mode

The ST7580 data link layer uses a CRC code for error detection. The data link layer

computes the CRC field and builds the frames for transmitting frames.

When receiving, it computes a CRC on the received data and compares it against the

received CRC: it accepts the incoming frame only if the two values are equal. A further

feature of reception configuration is the sniffer flag: if activated, the ST7580 notifies the host

about frames received with wrong CRC also.

10.3

Security services

The ST7580 is able to encrypt / decrypt frames (on DL payload) through algorithms based

on AES with 128-bit keys. A dedicated key that can be read and written by the external host

is used for both transmitting and receiving frames.