
118
Functional Operation
Chapter 3
AMD-8111 HyperTransport I/O Hub Data Sheet
24674
Rev. 3.00
April 2003
AMD Preliminary Information
3.10.13.2
Operation Without MII Management Interface
The Port Manager normally sets up the speed, duplex mode, and flow control (pause) ability of the
MAC based on the results of auto-negotiation. However, it is possible to operate the device with no
MII Management Interface connection, in which case the Port Manager is not able to start the auto-
negotiation process or set up the MAC based on auto-negotiation results. This may happen if the
network controller is connected to a multi-PHY device that has only one MII Management Interface
that is shared among several PHYs.
If the network controller is operating without a MII Management Interface connection to its external
PHY, the host CPU can force the MAC into the desired state by clearing the EN_PMGR bit in CMD3
Register to 0 to disable the Port Manager, then writing to the following bits:
1. FORCE_FULL_DUPLEX (CMD3, bit 12)
2. FORCE_LINK_STATUS (CMD3, bit 11)
3. Force Receive Pause Enable (FRPE, FLOW_CONTROL, bit 21)
4. Force Transmit Pause Enable (FTPE, FLOW_CONTROL, bit 22)
These bits set up the duplex mode, link status, and flow control ability in the MAC and put the MAC
into the Link Pass state.
3.10.14
Regulating Network Traffic
The network controller provides two hardware mechanisms for regulating network traffic: 802.3x
Flow Control and collision-based back pressure. 802.3x Flow Control applies to full-duplex operation
only, while back pressure applies to half-duplex operation only. 802.3x Flow Control works by
sending and receiving MAC Control PAUSE frames, which cause the receiving station to postpone
transmissions for a time determined by the contents of the PAUSE frame. Back pressure forces
collisions to occur when other nodes attempt to transmit, thereby preventing other nodes from
transmitting for periods of times determined by the back-off algorithm.
The device includes support for two styles of full-duplex flow control, Fixed Length Pause flow
control and Variable Length Pause flow control. In the Variable Length Pause style, which is similar
to an XON-XOFF protocol, a pause frame whose request_operand field (bytes 17 and 18) contains
0FFFFh is sent to prevent the link partner from transmitting. Later, a pause frame whose
request_operand field contains 0 is sent to allow the link partner to resume transmissions. The
Variable Length Pause style of flow control is selected by clearing the Fixed Length Pause bit (FIXP,
Flow Control Register, bit 18) to 0.
For the Fixed Length Pause style of flow control, a single pause frame is sent to halt transmissions for
a predetermined period of time. The contents of the request_operand field of this frame are taken from
the Pause Length register. This style of flow control is selected by setting the Fixed Length Pause bit
(FIXP) to 1.