NXP Semiconductors UM10237

User manual Rev. 02 — 19 December 2008 256 o f 792

Chapter 11: LPC24XX Ethernet

(0x7FE0 11F8) to the TxStatus register. The device driver writes the number of descriptors

and statuses minus 1(3) to the TxDescriptorNumber register. The descriptors and

statuses in the arrays need not be initialized, yet.

At this point, the transmit datapath may be enabled by setting the TxEnable bit in the

Command register. If the transmit datapath is enabled while there are no further frames to

send the TxFinishedInt interrupt flag will be set. To reduce the processor interrupt load

only the desired interrupts can be enabled by setting the relevant bits in the IntEnable

Now suppose application software wants to transmit a frame of 12 bytes using a TCP/IP

protocol (in real applications frames will be larger than 12 bytes). The TCP/IP stack will

add a header to the frame. The frame header need not be immediately in front of the

payload data in memory. The device driver can program the Tx DMA to collect header and

payload data. To do so, the device driver will program the first descriptor to point at the

frame header; the Last flag in the descriptor will be set to false/0 to indicate a

multi-fragment transmission. The device driver will program the next descriptor to point at

the actual payload data. The maximum size of a payload buffer is 2 kB so a single

descriptor suffices to describe the payload buffer. For the sake of the example thoug h the

payload is distributed across two descriptors. After the first descriptor in the array

describing the header, the second descriptor in the array describes the initial 8 bytes of

the payload; the third descriptor in the array describes the remaining 4bytes of the frame.

In the third descriptor the Last bit in the Control word is set to true/1 to indicate it is the last

descriptor in the frame. In this example the Interrupt bit in the descriptor Control field is set

in the last fragment of the frame in order to trigger an interrupt after the transmission

completed. The Size field in the descriptor’s Control word is set to the number of bytes in

the fragment buffer, -1 encoded.

Note that in real device drivers, the payload will typically only be split across multiple

descriptors if it is more than 2 kB. Also note that transmission payload data is forwarded to

the hardware without the device driver copying it (zero copy device driver).

After setting up the descriptors for the transaction the device driver increments the

TxProduceIndex register by 3 since three descriptors have been programmed. If the

transmit datapath was not enabled during initialization the device driver needs to enable

the datapath now.

If the transmit datapath is enabled the Ethernet block will start transmitting the frame as

soon as it detects the TxProduceIndex is not equal to TxConsumeIndex - both were zero

after reset. The Tx DMA will start reading the descriptors from memory. The memory

system will return the descriptors and the Ethernet block will accept them one by one

while reading the transmit data fragments.

As soon as transmission read data is returned from memory, the Ethernet block will try to

start transmission on the Ethernet connection via the (R)MII interface.

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.