UM0934 datasheet(29/113 Pages) STMICROELECTRONICS

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UM0934

MAC layer

Doc ID 17389 Rev 2

29/113

4.5.4

Transmission

When upper layers request a transmission through an M_Data.Request(DA, M_sdu), the

MAC builds the long frame header by copying the DA value into the destination address field

and by copying the local MAC address value into the source address field.

4.5.5

Group addresses

The MAC supports pre-defined group addresses (Table 6).

Further custom group addresses (up to 5) can be specified through MIB object 0005h (MAC

Group Addresses, Section 6.5.6).

4.6

Time-slot synchronization

As explained in Section 3.3, all the nodes belonging to the network must use the same time-

slot division in order to be able to communicate. The time-slot synchronization is first

achieved by the Client, by choosing an arbitrary time-slot start, while all the other nodes

(Server) acquire the Client’s time-slot division as described in following sections.

4.6.1

PHY layer

At PHY layer, the time-slot synchronization is acquired by a Server as soon as it receives

the PRE+SSD sequence. As the PHY layer does not perform any check on frame CRC and

addresses, the reception of any physical frame sent over the network, whatever the source

address and even if corrupted by noise, causes the Server to acquire the time-slot

synchronization (if not already synchronized).

Upon receipt of the PRE+SSD sequence, the PHY notifies to the MAC a P_Sync.Indication.

Table 6.

Pre-defined group addresses

Name

Value

Description

NO-BODY

000h

Reserved value, it can’t be assigned to any node in the network.

If a frame with DA equal to NO-BODY is received, a Client always generates

a M_Data.Indication, while a Server generates a M_Data.Indication only if at

least one MAC group addresses is equal to 0x000.

NEW

FFEh

Nodes with local MAC address equal to NEW.

NOT-NEW

FFCh Nodes with local MAC address different from NEW.

BROADCAST

FFFh

Broadcast address. All the network nodes generate an M_Data.Indication if

the DA field in the received MAC frame is equal to BROADCAST.

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