BCM1250/BCM1125/BCM1125H
User Manual
10/21/02
B r o a d c o m C o r p o r a t i o n
Page
266
Section 9: Ethernet MACs
Document
1250_1125-UM100CB-R
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ROTOCOL
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NGINE
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GMII/MII
The Ethernet MAC engine performs all the MAC layer processing needed to comply with the IEEE 802.3
standard at 10 Mbps, 100 Mbps and 1000 Mbps. It interfaces to the receive and transmit FIFOs to move data
into and out of the system, and connects to an external physical layer interface (PHY) using the standard MII
(media independent interface) connection for 10/100 Mbps operation and the GMII for gigabit operation.
Many of the protocol parameters are programmable. They must be set to the correct values for IEEE 802.3
operation, but may be changed in other applications.
As well as the basic protocol processing during transmission the engine can:
•
Automatically retry packets that experience transmission errors during the first few bytes.
•
Insert or remove a VLAN tag.
•
Append padding to short packets to meet the minimum frame size.
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Overwrite the source address field in the packet with the MAC's own source address.
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Append a CRC to the end of the packet.
During reception the protocol engine can:
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Automatically drop packets that have errors during their first few bytes.
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Filter incoming packets and automatically discard any with a destination address that does not match.
Both exact and hash-based matching are available.
•
Flag errors and packet type information in the DMA descriptor.
The protocol engine can run the 10 Mbps, 100 Mbps and 1000 Mbps protocol variants and supports full and
half-duplex operation at all speeds. Typically the external PHY would support auto-negotiation of the line
parameters. During initialization the software will read the negotiated state using the management interface to
the PHY and configure the MAC accordingly.
The protocol engine implements flow control. It will back-pressure the link when software explicitly requests or
when the number of buffers available to the receive DMA engine falls below a threshold. In half-duplex mode
back-pressure is applied by forcing collisions or a fake carrier on the line, in full-duplex mode flow control
(pause) frames are sent.