pnx1700 NXP Semiconductors, pnx1700 Datasheet - Page 734

• Manufacturer Part Number: pnx1700
• Description: Connected Media Processor
• Manufacturer: NXP Semiconductors

Philips Semiconductors
Volume 1 of 1
PNX17XX_SER_1
Preliminary data sheet
Figure 12: Transmit flow control
Tx clk cycles
MirrorCounter
(1/515 bit slots)
MII Receive
MMIO
MII Transmit
0
5.11 Half-duplex Mode Back Pressure
MirrorCounter,
PauseTimer,
TxFlowCtrl
Transmission
Normal
50
Transmit Flow Control Example
Figure 12
In this example, the LAN100 receives a packet while transmitting another packet
(operating in full duplex.) The device driver detects 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.
In response to enabling flow control, the LAN100 will send out a pause control frame
on the LAN after the packet currently being transmitted has finished. 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 Datapath will continue normal
transmissions.
As soon as software disables transmit flow control, a zero-pause control frame is
transmitted to resume the receive process.
In half-duplex mode, the LAN100 can generate back pressure to stall receive packets
by sending a continuous preamble that basically jams any other transmissions on the
Ethernet medium. When the Ethernet module operates in half-duplex mode,
asserting the TxFlowControl bit in the Command register will cause continuous
preamble to be applied on the Ethernet wire, effectively blocking traffic from any other
Ethernet station on the same segment.
Receive
Receive
Normal
Normal
Transmission
Control
Pause
Frame
100
illustrates the transmit flow control.
150
Rev. 1 — 17 March 2006
Transmission
Normal
Chapter 23: LAN100 — Ethernet Media Access Controller
200
250
Pause
Transmission
Control
Frame
Pause
300
TxFlowCtrl
Clear
350
Transmission
© Koninklijke Philips Electronics N.V. 2006. All rights reserved.
Control
Pause
Frame
PNX17xx Series
400
450
Receive
Normal
500
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