
DS33Z41 Quad IMUX Ethernet Mapper
8.12.1 Full-Duplex Flow Control
Automatic flow control is enabled by default. The host processor can disable this functionality with SU.GCR.ATFLOW. The flow control mechanism is governed by the high watermarks (SU.RQHT). The SU.RQLT low threshold can be used as indication that the network congestion is clearing up. The value of SU.RQLT does not affect the flow control. When the connection queue high threshold is exceeded the DS33Z41 will send a pause frame with the timer value programmed by the user. See Table
The pause frame causes the distant transmitter to “pause for a time” before starting transmission again. The pause command has a multicast address
The user has the option of not enabling automatic flow control. In this case the thresholds and corresponding interrupt mechanism to send pause frame by writing to flow control busy bit in the MAC flow control registers SU.MACFCR.FCB, SU.GCR.JAME, and SU.MACFCR. This allows the user to set not only the watermarks but also to decide when to send a pause frame or not based on watermark crossings.
On the receive side the user has control over whether to respond to the pause frame sent by the distant end (PCF bit). Note that if automatic flow control is enabled the user cannot modify the FCE bit in the MAC flow control register. On the Transmit queue the user has the option of setting high and low thresholds and corresponding interrupts. There is no automatic flow control mechanism for data received from the Serial side waiting for transmission over the Ethernet interface during times of heavy Ethernet congestion.
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