
Chapter 3
Functional Operation
87
24674
Rev. 3.00
April 2003
AMD-8111 HyperTransport I/O Hub Data Sheet
AMD Preliminary Information
The MAC engine allows the user to program both the IPG and the first part deferral (IFS1) through
the IFS and IFS1 registers. The user can change the IPG value from its default of 96-bit times to
compensate for delays through the external PHY device. Changing IFS1 alters the period for which
the MAC engine defers to incoming receive frames.
Note:
Care must be exercised when altering these parameters. Undesirable network activity could
result!
This transmit two-part deferral algorithm is implemented as an option which can be disabled using
the DXMT2PD bit (CMD2, bit 10). When DXMT2PD is set to 1, the IFS1 register is ignored, and the
value 0 is used for the Inter FrameSpacingPart1 parameter. However, the IPG value is still valid.
When the MAC engine device operates in full-duplex mode, the IPG timer starts counting when
TX_EN is de-asserted. CRS is ignored in full-duplex mode.
3.10.4.3.2 Signal Quality Error (SQE) Test
During the time period immediately after a transmission has been completed, an external transceiver
operating in the 10 Mb/s half-duplex mode should generate an SQE Test signal on the COL pin within
0.6
μ
s to 1.6
μ
s after the transmission ceases. Therefore, when the network controller is operating in
half-duplex mode, the IPG counter ignores the COL signal during the first 40-bit times of the inter-
packet gap. This 40-bit times is the time period in which the SQE Test message is expected.
The SQE Test was originally designed to check the integrity of the Collision Detection mechanism
independently of the Transmit and Receive capabilities of the Physical Layer. However, MII-based
PHY devices detect collisions by sensing receptions that occur during transmissions, a process that
does not require a separate level-sensing collision detection mechanism. Collision detection is
therefore dependent on the health of the receive channel. Since the Link Monitor function checks the
health of the receive channel, the SQE test is not very useful for MII-based devices. Therefore, the
device does not report or count SQE Test failures.
3.10.4.3.3 Collision Handling
Collision detection is performed and reported to the MAC engine through the COL input pin. Since
the COL signal is not required to be synchronized with TX_CLK when the device is operating in half-
duplex mode, the COL signal must be asserted for at least three TX_CLK cycles in order to be
detected reliably.
If a collision is detected before the complete preamble/SFD sequence has been transmitted, the MAC
engine completes the preamble/SFD before appending the jam sequence. If a collision is detected
after the preamble/SFD has been completed, but before 512 bits have been transmitted, the MAC
engine aborts the transmission and appends the jam sequence immediately. The jam sequence is a 32-
bit all zeros pattern.
The MAC engine attempts to transmit a frame a total of 16 times (initial attempt plus 15 retries) due
to normal collisions (those within the slot time). Detection of collision causes the transmission to be
rescheduled to a time determined by the random backoff algorithm. If a single retry was required, the