DS33Z41 Quad IMUX Ethernet Mapper
Figure 8-6. Flow Control Using Pause Control Frame
Receive Queue
Growth
8
Rx
Data
Receive Queue Low
Water
Receive Queue High
Water Mark
Initiate Flow control
8.12.2 Half-Duplex Flow control
Half duplex flow control uses a jamming sequence to exert backpressure on the transmitting node. The receiving node jams the first 4 bytes of a packet that are received from the MAC in order to cause collisions at the distant end. In both 100Mbps and 10Mbps MII/RMII modes, 4 bytes are jammed upon reception of a new frame. Note that the jamming mechanism does not jam the current frame that is being received during the watermark crossing, but will wait to jam the next frame after the SU.RQHT bit is set. If the queue remains above the high threshold, received frames will continue to be jammed. This jam sequence is stopped when the queue falls below the high threshold.
8.12.3 Host-Managed Flow control
Although automatic flow control is recommended, flow control by the host processor is also possible. By utilizing the high watermark interrupts, the host processor can manually issue pause frames or jam incoming packets to exert backpressure on the transmitting node. Pause frames can be initiated with SU.MACFCR.FCB bit. Jam sequences can be initiated be setting SU.GCR.JAME. The host can detect pause frames by monitoring SU.RFSB3.UF and SU.RFSB3.CF. Jammed frames will be indistinguishable from packet collisions.
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