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IXF1104 4-Port Gigabit Ethernet Media Access Controller
Datasheet 70
Document Number: 278757
Revision Number: 009
Revision Date: 27-Oct-2005
The IXF1104 MAC implements the IEEE 802.3x standard RX FIFO threshold-based Flow Control
in copper and fiber modes. When appropriately programmed, the MAC can both generate and
respond to IEEE standard pause frames in full-duplex operation. The IXF1104 MAC also supports
externally triggered flow control through the Transmit Pause Control interface.
In half-duplex operation, the MAC generates collisions instead of sending pause frames to manage
the incoming traffic from the link partner
5.1.2.1 802.3x Flow Control (Full-Duplex Operation)
The IEEE 802.3x standard identifies four options related to system flow control:
• No Pause
• Symmetric Pause (both directions)
• Asymmetric Pause (Receive direction only)
• Asymmetric Pause (Transmit direction only)
The IXF1104 supports all four options on a per-port basis. Bits 2:0 of the “FC Enable ($ Port_Index
+ 0x12)” on page 168 provide programmable control for enabling or disabling flow control in each
direction independently.
The IEEE 802.3x flow control mechanism is accomplished within the MAC sublayer, and is based
on RX FIFO thresholds called watermarks. The RX FIFO level rises and falls as packets are
received and processed. When the RX FIFO reaches a watermark (either exceeding a High or
dropping below a Low after exceeding a High), the IXF1104 control sublayer signals an internal
state machine to transmit a PAUSE frame. The FIFOs automatically generate PAUSE frames (also
called control frames) to initiate the following:
• Halt the link partner when the High watermark is reached.
• Restart the link partner when the data stored in the FIFO falls below the Low watermark.
Figure 7 illustrates the IEEE 802.3 FIFO flow control functions.