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Functional Operation
Chapter 3
AMD-8111 HyperTransport I/O Hub Data Sheet
24674
Rev. 3.00
April 2003
AMD Preliminary Information
3.10.5.4
Conduit Mode
The Conduit Mode of operation allows the LAN Ethernet controller to pass frame data to an external
MAC-type device attached to the MII bus. This option eliminates the need for a frame buffer in the
external device. In this mode the backoff logic is disabled, and the COL and CRS signals from the
MII are used to control transmit retry and frame discard. The internal synchronizers on the COL and
CRS inputs are bypassed, and the external device must provide COL and CRS inputs that are
synchronized with TX_CLK.
The CRS signal is used for flow control across the MII bus. The external device controls the local
transmit MAC by asserting CRS after the local device asserts TX_EN. CRS is held on until the
external device has finished processing the frame, preventing the local device from starting the
transmission of the next frame. The current transmit frame is retained in the FIFO until CRS
deasserts. If COL is asserted at any time while CRS is asserted, the frame is requeued and
retransmitted.
In Conduit Mode all MIB counters work like they do in half duplex mode, except that the
XmtExcessiveCollision counter is incremented when COL is asserted while CRS is deasserted (as
explained below), and not when a frame has experienced 16 unsuccessful transmission attempts.
Conduit Mode is enabled by setting the CONDUIT_MODE bit (CMD2, bit 29).
In Conduit Mode the MII handshake is modified as follows:
1. The CRS and COL inputs must be synchronized with TX_CLK, changing state at the falling edge
of TX_CLK. In this mode CRS and COL must meet set-up and hold timing requirements with
respect to the rising edge of TX_CLK similar to the timing requirements for the synchronous
signals in the MII receive path.
2. If COL is asserted while CRS is also asserted, the frame is requeued for transmission. The frame
is not dropped when CRS is deasserted.
3. If COL is not asserted during the time that CRS is asserted, the appropriate MIB counters are
incremented to indicate a successful transmission, and the frame is discarded from memory when
CRS is deasserted.
4. If COL is asserted during the same TX_CLK cycle in which CRS is deasserted, the frame is
discarded and counted as an excessive collision error. (The XmtExcessiveCollision counter is
incremented.) In this case CRS must be held Low for at least 2 TX_CLK cycles. After 2 TX_CLK
cycles, CRS may be asserted again to delay the transfer of the next frame over the MII.
5. Following deassertion of CRS, normal IPG is counted and next frame transmitted. The external
device asserts CRS when it sees TX_EN asserted.
Figure 19 shows a frame that is requeued and then successfully transmitted. Figure 20 shows a frame
that is dropped because the external device reports excessive collisions. In this figure a second frame
is transmitted successfully with no collisions.