
88
Functional Operation
Chapter 3
AMD-8111 HyperTransport I/O Hub Data Sheet
24674
Rev. 3.00
April 2003
AMD Preliminary Information
XmtOneCollision counter is incremented. If more than one retry was required, the
XmtMultipleCollision counter is incremented. If all 16 attempts experienced collisions, the
XmtExcessiveCollision counter is incremented. After an excessive collision error, the transmit
message is flushed from the FIFO.
If retries have been disabled by setting the DRTY bit in the CMD2 register, the MAC engine
abandons transmission of the frame on detection of the first collision. In this case,
XmtExcessiveCollision counter is incremented, and the transmit message is flushed from the FIFO.
If a collision is detected after 512-bit times have been transmitted, the collision is termed a late
collision. The MAC engine aborts the transmission, appends the jam sequence, and increments the
XmtLateCollision counter. No retry attempt is scheduled on detection of a late collision. The transmit
message is flushed from the FIFO.
The IEEE 802.3 Standard requires use of a “truncated binary exponential backoff” algorithm, which
provides a controlled pseudo random mechanism to enforce the collision backoff interval, before
retransmission is attempted. For details, see IEEE standard 802.3, 1998 Edition, 4.2.3.2.5.
The MAC engine provides an alternative algorithm, which suspends the counting of the slot time/IPG
during the time that receive carrier sense is detected. This aids in networks where large numbers of
nodes are present, and numerous nodes can be in collision. It effectively accelerates the increase in
the backoff time in busy networks and allows nodes not involved in the collision to access the
channel, while the colliding nodes await a reduction in channel activity. Once channel activity is
reduced, the nodes resolving the collision time-out their slot time counters as normal.
This modified backoff algorithm is enabled when EMBA (CMD2, bit 11) is set to 1.
3.10.5
Transmit Operation
The transmit operation and features of the network controller are controlled by programmable
options. The controller provides a large transmit FIFO to provide frame buffering for increased
system latency, automatic retransmission with no FIFO reload, and automatic transmit padding.
3.10.5.1
Transmit Function Programming
Automatic transmit features such as retry on collision, FCS generation/transmission, and pad field
insertion can all be programmed to provide flexibility in the (re-) transmission of messages.
Automatic pad field insertion is controlled by the APAD_XMT bit in the CMD2 register.
The FCS generation/transmission feature can be programmed as a static feature (DXMTFCS bit in
CMD2) or dynamically on a frame-by-frame basis (ADD_FCS bit in the transmit descriptor).
REX_UFLO (CMD2, bit 1) can be programmed to cause the transmitter to automatically restart the
transmission process instead of discarding a frame that experiences an underflow error. After an
underflow error the retransmission does not begin until the entire frame has been loaded into the
transmit FIFO.