
108
Functional Operation
Chapter 3
AMD-8111 HyperTransport I/O Hub Data Sheet
24674
Rev. 3.00
April 2003
AMD Preliminary Information
TX_ER is not driven by the network controller and therefore not looped back.
Internal loopback should not be used on a live network because transmit data path signals might
interfere with network traffic. The PHY should be disconnected from the network or put into the
“Isolate” state before internal loopback is used.
3.10.10
Full-Duplex Operation
When the network controller is in full-duplex mode, it can receive a frame from the network at the
same time that it is transmitting a frame. Full-duplex operation is normally enabled through the auto-
negotiation process. Alternatively full-duplex operation can be enabled by clearing the EN_PMGR bit
in CMD3 Register to 0 to disable the Port Manager and then setting the FORCE_FULL_DUPLEX bit
(CMD3, bit 12).
In full-duplex mode the network controller behaves the same way it does in half-duplex mode with
the following exceptions:
Transmission is not deferred while receive is active.
The IPG counter which governs transmit deferral during the IPG between back-to-back transmits
is started when transmit activity for the first packet ends, instead of when transmit and carrier
activity ends.
The collision indication input to the MAC engine is ignored.
3.10.11
External PHY Interface
The network controller supports the Media Independent Interface (MII) as defined by the IEEE 802.3
standard. This Reconciliation Sublayer interface allows a variety of PHYs (100BASE-TX, 100BASE-
FX, 100BASE-T4, 100BASE-T2, 10BASE-T, etc.) to be attached to the MAC engine without future
upgrade problems. The MII interface is a 4-bit (nibble) wide data path interface that runs at 25 MHz
for 100-Mbit/s networks or 2.5 MHz for 10-Mbit/s networks. The interface consists of two
independent data paths, receive (RXD(3:0)) and transmit (TXD(3:0)), control signals for each data
path (RX_ER, RX_DV, TX_EN), network status signals (COL, CRS), clocks (RX_CLK, TX_CLK)
for each data path, and a two-wire management interface (MDC and MDIO). See Figure 25 on page
109.
The transmit and receive paths in the controller's MAC are independent. The TX_CLK and RX_CLK
need not run at the same frequency. TX_CLK can slow down or stop without affecting receive and
vice versa. It is only necessary to respect the minimum clock High and Low time specifications when
switching TX_CLK or RX_CLK. This facilitates operation with PHYs that use MII signaling but do
not adhere to 802.3 MII specifications.
3.10.11.1
MII Transmit Interface
The MII transmit clock is generated by the external PHY and is sent to the network controller on the
TX_CLK input pin. The clock can run at 25 MHz or 2.5 MHz, depending on the speed of the network
to which the external PHY is attached. The data is a nibble-wide (4 bits) data path, TXD(3:0), from