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DS33R11 Ethernet Mapper with Integrated T1/E1/J1 Transceiver
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Figure 9-3. Flow Control Using Pause Control Frame
Receive Queue
Growth
Receive Queue High
Water Mark
Initiate Flow control
8
Rx
Data
Receive Queue Low
Water
9.13.2 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 100Mbit/s and 10Mbit/s 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.
9.13.3 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.