UM10360 All information provided in this document is subject to legal disclaimers. © NXP B.V. 2013. All rights reserved.
User manual Rev. 3 — 19 December 2013 146 of 841
NXP Semiconductors
UM10360
Chapter 10: LPC176x/5x Ethernet
Hardware in the DMA engine controls how data incoming from the Ethernet MAC is saved
to memory, causes fragment related status to be saved, and advances the hardware
receive pointer for incoming data. Driver software must handle the disposition of received
data, changing of descriptor data addresses (to avoid unnecessary data movement), and
advancing the software receive pointer. The two pointers create a circular queue in the
descriptor array and allow both the DMA hardware and the driver software to know which
descriptors (if any) are available for their use, including whether the descriptor array is
empty or full.
Similarly, driver software must set up pointers to data that will be transmitted by the
Ethernet MAC, giving instructions for each fragment of data, and advancing the software
transmit pointer for outgoing data. Hardware in the DMA engine reads this information and
sends the data to the Ethernet MAC interface when possible, updating the status and
advancing the hardware transmit pointer.
10.7 Ethernet Packet
Figure 18 illustrates the different fields in an Ethernet packet.
A packet consists of a preamble, a start-of-frame delimiter and an Ethernet frame.
Fig 18. Ethernet packet fields
OPTIONAL
VLAN
SOURCE
ADDRESS
DesA
1st
octet
DesA
2nd
octet
DesA
3rd
octet
DesA
4th
octet
DesA
5th
octet
DesA
6th
octet
1st
octet
2nd
octet
3rd
octet
4th
octet
5th
octet
6th
octet
SrcA SrcA SrcA SrcA SrcA SrcA
LSB
bit 0
bit 1 bit 2 bit 3 bit 4 bit 5 bit 6
MSB
bit 7
DESTINATION
ADDRESS
PAYLOAD FCS
ETHERNET FRAME
PREAMBLE
7 bytes
ethernet packet
start-of-frame
delimiter
1 byte
time
LEN
TYPE
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