NXP Semiconductors LPC24XX UM10237 - Manual (Page 265)

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User manual Rev. 04 — 26 August 2009 265 of 792
NXP Semiconductors
UM10237
Chapter 11: LPC24XX Ethernet
The destination address and source address hash CRCs being written in the
StatusHashCRC word are the nine most significant bits of the 32 bit CRCs as calculated
by the CRC calculator.
9.10 Duplex modes
The Ethernet block can operate in full duplex and half duplex mode. Half or full duplex
mode needs to be configured by the device driver software during initialization.
For a full duplex connection the FullDuplex bit of the Command register needs to be set to
1 and the FULL-DUPLEX bit of the MAC2 configuration register needs to be set to 1; for
half duplex the same bits need to be set to 0.
9.11 IEE 802.3/Clause 31 flow control
Overview
For full duplex connections, the Ethernet block supports IEEE 802.3/clause 31 flow control
using pause frames. This type of flow control may be used in full-duplex point-to-point
connections. Flow control allows a receiver to stall a transmitter e.g. when the receive
buffers are (almost) full. For this purpose, the receiving side sends a pause frame to the
transmitting side.
Pause frames use units of 512 bit times corresponding to 128 rx_clk/tx_clk cycles.
Receive flow control
In full-duplex mode, the Ethernet block will suspend its transmissions when the it receives
a pause frame. Rx flow control is initiated by the receiving side of the transmission. It is
enabled by setting the ‘RX FLOW CONTROL’ bit in the MAC1 configuration register. If the
RX FLOW CONTROL’ bit is zero, then the Ethernet block ignores received pause control
frames. When a pause frame is received on the Rx side of the Ethernet block,
transmission on the Tx side will be interrupted after the currently transmitting frame has
completed, for an amount of time as indicated in the received pause frame. The transmit
datapath will stop transmitting data for the number of 512 bit slot times encoded in the
pause-timer field of the received pause control frame.
By default the received pause control frames are not forwarded to the device driver. To
forward the receive flow control frames to the device driver, set the ‘PASS ALL RECEIVE
FRAMES’ bit in the MAC1 configuration register.
Transmit flow control
If case device drivers need to stall the receive data e.g. because software buffers are full,
the Ethernet block can transmit pause control frames. Transmit flow control needs to be
initiated by the device driver software; there is no IEEE 802.3/31 flow control initiated by
hardware, such as the DMA managers.
With software flow control, the device driver can detect a situation in which the process of
receiving frames needs to be interrupted by sending out Tx pause frames. Note that due
to Ethernet delays, a few frames can still be received before the flow control takes effect
and the receive stream stops.