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UM10360_0
© NXP B.V. 2009. All rights reserved.
User manual
Rev. 00.06 — 5 June 2009
167 of 808
NXP Semiconductors
UM10360
Chapter 10: LPC17xx Ethernet
In this example, a frame is received while transmitting another frame (full duplex.) The
device driver detects that some buffer might overrun and enables the transmit flow control
by programming the PauseTimer and MirrorCounter fields of the FlowControlCounter
register, after which it enables the transmit flow control by setting the TxFlowControl bit in
the Command register.
As a response to the enabling of the flow control a pause control frame will be sent after
the currently transmitting frame has been transmitted. When the pause frame
transmission completes the internal mirror counter will start counting bit slots; as soon as
the counter reaches the value in the MirrorCounter field another pause frame is
transmitted. While counting the transmit data path will continue normal transmissions.
As soon as software disables transmit flow control a zero pause control frame is
transmitted to resume the receive process.
17.9 Half-Duplex mode backpressure
When in half-duplex mode, backpressure can be generated to stall receive packets by
sending continuous preamble that basically jams any other transmissions on the Ethernet
medium. When the Ethernet block operates in half duplex mode, asserting
the TxFlowControl bit in the Command register will result in applying continuous preamble
on the Ethernet wire, effectively blocking traffic from any other Ethernet station on the
same segment.
In half duplex mode, when the TxFlowControl bit goes high, continuous preamble is sent
until TxFlowControl is de-asserted. If the medium is idle, the Ethernet block begins
transmitting preamble, which raises carrier sense causing all other stations to defer. In the
event the transmitting of preamble causes a collision, the backpressure ‘rides through’ the
collision. The colliding station backs off and then defers to the backpressure. If during
backpressure, the user wishes to send a frame, the backpressure is interrupted, the frame
sent and then the backpressure resumed. If TxFlowControl is asserted for longer than
3.3 ms in 10 Mbps mode or 0.33 ms in 100 Mbps mode, backpressure will cease sending
preamble for several byte times to avoid the jabber limit.
Fig 21. Transmit Flow Control
MirrorCounter
(1/515 bit
slots)
400
0
150
300
200
450
350
250
50
100
500
PauseTimer
MirrorCounter
TxFlowCtl
clear
TxFlowCtl
pause control
frame
transmission
pause control
frame
transmission
pause control
frame
transmission
normal transimisson
normal receive
normal
transmission
normal receive
RMII
receive
RMII
transmit
device driver
register
writes
pause in effect