NXP Semiconductors LPC24XX UM10237 - Manual (Page 247)

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User manual Rev. 04 — 26 August 2009 247 of 792
NXP Semiconductors
UM10237
Chapter 11: LPC24XX Ethernet
For multi-fragment frames, the value of the LateCollision, ExcessiveCollision,
ExcessiveDefer, Defer and CollissionCount bits in all but the last fragment in the frame will
be 0. The status of the last fragment in the frame will copy the value for these bits from the
MAC. All fragment statuses will have valid Error, NoDescriptor and Underrun bits.
9. Ethernet block functional description
This section defines the functions of the DMA capable 10/100 Ethernet MAC. After
introducing the DMA concepts of the Ethernet block, and a description of the basic
transmit and receive functions, this section elaborates on advanced features such as flow
control, receive filtering, etc.
9.1 Overview
The Ethernet block can transmit and receive Ethernet packets from an off-chip Ethernet
PHY connected through the MII or RMII interface. MII or RMII mode can be selected from
software.
Typically during system start-up, the Ethernet block will be initialized. Software
initialization of the Ethernet block should include initialization of the descriptor and status
arrays as well as the receiver fragment buffers.
To transmit a packet the software driver has to set up the appropriate Control registers
and a descriptor to point to the packet data buffer before transferring the packet to
hardware by incrementing the TxProduceIndex register. After transmission, hardware will
increment TxConsumeIndex and optionally generate an interrupt.
The hardware will receive packets from the PHY and apply filtering as configured by the
software driver. While receiving a packet the hardware will read a descriptor from memory
to find the location of the associated receiver data buffer. Receive data is written in the
Table 241. Transmit status information word
Bit Symbol Description
20:0 - Unused
24:21 CollisionCount The number of collisions this packet incurred, up to the
Retransmission Maximum.
25 Defer This packet incurred deferral, because the medium was occupied.
This is not an error unless excessive deferral occurs.
26 ExcessiveDefer This packet incurred deferral beyond the maximum deferral limit and
was aborted.
27 ExcessiveCollision Indicates this packet exceeded the maximum collision limit and was
aborted.
28 LateCollision An Out of window Collision was seen, causing packet abort.
29 Underrun A Tx underrun occurred due to the adapter not producing transmit
data.
30 NoDescriptor The transmit stream was interrupted because a descriptor was not
available.
31 Error An error occurred during transmission. This is a logical OR of
Underrun, LateCollision, ExcessiveCollision, and ExcessiveDefer.