Figure 19-10. Pause Operation State Machine - IBM PowerPC 405GP User Manual

the MAC control packets to MAL unless the Propagate Pause Packets bit is set in the Receive mode
(EMACO_RMR[PPP] = 1).
Valid Control (Pause) packet is detected
Wait for transmission completion
Currently ongoing transmit is completed without interruption
Pause function Wait for Pause Counter expiration
End pause Open the transmit path
Figure 19·10. Pause Operation State Machine
In the Pause Function state, EMAC decrements its internal pause timer, which was set to the Timer
Value field of the received control packet.
Note 1: The transmission of control (pause) packets is not affected by the reception of a receive
control (pause) packet. Received control (pause) packets inhibit only the transmission of
regular packets from the Transmit FIFO.
Note 2: Receipt of a new valid control (pause) packet causes the pause timer of EMAC to be
reloaded with the contents of Timer Value field of the recently received packet, regardless of
the current pause timer setting. This indicates new pause operations take precedence over
earlier pause operations.
19.6
VLAN Support
EMAC can handle VLAN tagged packets, as specified in IEEE Draft P802.3ac/D1.0a standard when
EMACO_MR1 [VLE] = 1.
A Tagged MAC Frame is an extension of the standard MAC packet. The extension for VLAN tag
support consists of a 4-octet VLAN tag inserted between the end of the Source Address and the
beginning of the Length/Type fields of the MAC packet.
The VLAN tag consists of two fields:
• A 2-octet constant Type field value equal to the VLAN Tag Protocol Identifier (Ox81 00)
• A 2-octet field containing Tag Control Information (TCI)
The MAC Client Data and FCS fields of the basic MAC packet follow the VLAN tag. The length of the
packet is extended by four octets by the VLAN tag (up to 1522 bytes). The FCS is calculated over all
fields from the Destination Address through the end of the MAC client data or Pad (if present); that is,
all fields except the preamble, SPD, and FCS.
19-18 PPC405GP User's Manual Preliminary