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 173 of 841
NXP Semiconductors
UM10360
Chapter 10: LPC176x/5x Ethernet
10.15 Descriptor and status formats
This section defines the descriptor format for the transmit and receive scatter/gather DMA
engines. Each Ethernet frame can consist of one or more fragments. Each fragment
corresponds to a single descriptor. The DMA managers in the Ethernet block scatter (for
receive) and gather (for transmit) multiple fragments for a single Ethernet frame.
10.15.1 Receive descriptors and statuses
Figure 19 depicts the layout of the receive descriptors in memory.
Receive descriptors are stored in an array in memory. The base address of the array is
stored in the RxDescriptor register, and should be aligned on a 4 byte address boundary.
The number of descriptors in the array is stored in the RxDescriptorNumber register using
a minus one encoding style e.g. if the array has 8 elements the register value should be 7.
Parallel to the descriptors there is an array of statuses. For each element of the descriptor
array there is an associated status field in the status array. The base address of the status
array is stored in the RxStatus register, and must be aligned on an 8 byte address
boundary. During operation (when the receive data path is enabled) the RxDescriptor,
RxStatus and RxDescriptorNumber registers should not be modified.
Two registers, RxConsumeIndex and RxProduceIndex, define the descriptor locations
that will be used next by hardware and software. Both registers act as counters starting at
0 and wrapping when they reach the value of RxDescriptorNumber. The RxProduceIndex
contains the index of the descriptor that is going to be filled with the next frame being
Fig 19. Receive descriptor memory layout
1
2
3
4
5
StatusInfo
StatusHashCRC
StatusInfo
StatusHashCRC
StatusInfo
StatusHashCRC
StatusInfo
StatusHashCRC
StatusInfo
StatusHashCRC
StatusInfo
StatusHashCRC
PACKET
CONTROL
PACKET
CONTROL
PACKET
CONTROL
PACKET
CONTROL
PACKET
CONTROL
PACKET
CONTROL
RxStatus
RxDescriptorNumber
RxDescriptor
DATA BUFFER
DATA BUFFER
DATA BUFFER
DATA BUFFER
DATA BUFFER
DATA BUFFER
To Next Page
Loading...
