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 209 of 841
NXP Semiconductors
UM10360
Chapter 10: LPC176x/5x Ethernet
10.18 AHB bandwidth
The Ethernet block is connected to an AHB bus which must carry all of the data and
control information associated with all Ethernet traffic in addition to the CPU accesses
required to operate the Ethernet block and deal with message contents.
10.18.1 DMA access
Assumptions
By making some assumptions, the bandwidth needed for each type of AHB transfer can
be calculated and added in order to find the overall bandwidth requirement.
The flexibility of the descriptors used in the Ethernet block allows the possibility of defining
memory buffers in a range of sizes. In order to analyze bus bandwidth requirements,
some assumptions must be made about these buffers. The "worst case" is not addressed
since that would involve all descriptors pointing to single byte buffers, with most of the
memory occupied in holding descriptors and very little data. It can easily be shown that
the AHB cannot handle the huge amount of bus traffic that would be caused by such a
degenerate (and illogical) case.
For this analysis, an Ethernet packet is assumed to consist of a 64 byte frame.
Continuous traffic is assumed on both the transmit and receive channels.
This analysis does not reflect the flow of Ethernet traffic over time, which would include
inter-packet gaps in both the transmit and receive channels that reduce the bandwidth
requirements over a larger time frame.
Types of DMA access and their bandwidth requirements
The interface to an external Ethernet PHY is via RMII. RMII operates at 50 MHz,
transferring a byte in 4 clock cycles. The data transfer rate is 12.5 Mbps.
The Ethernet block initiates DMA accesses for the following cases:
• Tx descriptor read:
– Transmit descriptors occupy 2 words (8 bytes) of memory and are read once for
each use of a descriptor.
– Two word read happens once every 64 bytes (16 words) of transmitted data.
– This gives 1/8th of the data rate, which = 1.5625 Mbps.
• Rx descriptor read:
– Receive descriptors occupy 2 words (8 bytes) of memory and are read once for
each use of a descriptor.
– Two word read happens once every 64 bytes (16 words) of received data.
– This gives 1/8th of the data rate, which = 1.5625 Mbps.
• Tx status write:
– Transmit status occupies 1 word (4 bytes) of memory and is written once for each
use of a descriptor.
– One word write happens once every 64 bytes (16 words) of transmitted data.
– This gives 1/16th of the data rate, which = 0.7813 Mbps.
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