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 189 of 841
NXP Semiconductors
UM10360
Chapter 10: LPC176x/5x Ethernet
After transmitting each fragment of the frame the Tx DMA will write the status of the
fragment’s transmission. Statuses for all but the last fragment in the frame will be written
as soon as the data in the frame has been accepted by the Tx DMA manager. The status
for the last fragment in the frame will only be written after the transmission has completed
on the Ethernet connection.
Since the Interrupt bit in the descriptor of the last fragment is set, after committing the
status of the last fragment to memory the Ethernet block will trigger a TxDoneInt interrupt,
which triggers the device driver to inspect the status information.
In this example the device driver cannot add new descriptors as long as the Ethernet
block has not incremented the TxConsumeIndex because the descriptor array is full (even
though one descriptor is not programmed yet). Only after the hardware commits the status
for the first fragment to memory and the TxConsumeIndex is set to 1 by the DMA manager
can the device driver program the next (the fourth) descriptor. The fourth descriptor can
already be programmed before completely transmitting the first frame.
In this example the hardware adds the CRC to the frame. If the device driver software
adds the CRC, the CRC trailer can be considered another frame fragment which can be
added by doing another gather DMA.
Each data byte is transmitted across the RMII interface as four 2-bit values. The Ethernet
block adds the preamble, frame delimiter leader, and the CRC trailer if hardware CRC is
enabled. Once transmission on the RMII interface commences the transmission cannot
be interrupted without generating an underrun error, which is why descriptors and data
read commands are issued as soon as possible and pipelined.
Using an RMII PHY, the data communication between the Ethernet block and the PHY is
communicated at 50 MHz. In 10 Mbps mode data will only be transmitted once every 10
clock cycles.
10.17.4 Receive process
This section outlines the receive process including the activities in the device driver
software.
Device driver sets up descriptors
After initializing the receive descriptor and status arrays to receive frames from the
Ethernet connection, the receive data path should be enabled in the MAC1 register and
the Control register.
During initialization, each Packet pointer in the descriptors is set to point to a data
fragment buffer. The size of the buffer is stored in the Size bits of the Control field of the
descriptor. Additionally, the Control field in the descriptor has an Interrupt bit. The Interrupt
bit allows generation of an interrupt after a fragment buffer has been filled and its status
has been committed to memory.
After the initialization and enabling of the receive data path, all descriptors are owned by
the receive hardware and should not be modified by the software unless hardware hands
over the descriptor by incrementing the RxProduceIndex, indicating that a frame has been
received. The device driver is allowed to modify the descriptors after a (soft) reset of the
receive data path.
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