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
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