NXP Semiconductors LPC24XX UM10237 - Manual (Page 254)

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User manual Rev. 04 — 26 August 2009 254 of 792
NXP Semiconductors
UM10237
Chapter 11: LPC24XX Ethernet
Each time the Tx DMA manager commits a status word to memory it completes the
transmission of a descriptor and it increments the TxConsumeIndex (taking wrap around
into account) to hand the descriptor back to the device driver software. Software can
re-use the descriptor for new transmissions after hardware has handed it back.
The device driver software can keep track of the progress of the DMA manager by reading
the TxConsumeIndex register to see how far along the transmit process is. When the Tx
descriptor array is emptied completely, the TxConsumeIndex register retains its last value.
Write transmission status
After the frame has been transmitted over the (R)MII bus, the StatusInfo word of the frame
descriptor is updated by the DMA manager.
If the descriptor is for the last fragment of a frame (or for the whole frame if there are no
fragments), then depending on the success or failure of the frame transmission, error
flags (Error, LateCollision, ExcessiveCollision, Underrun, ExcessiveDefer, Defer) are set
in the status. The CollisionCount field is set to the number of collisions the frame incurred,
up to the Retransmission Maximum programmed in the Collision window/retry register of
the MAC.
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. Even if the descriptor is for a frame
fragment other than the last fragment, the error flags are returned via the AHB interface. If
the Ethernet block detects a transmission error during transmission of a (multi-fragment)
frame, all remaining fragments of the frame are still read via the AHB interface. After an
error, the remaining transmit data is discarded by the Ethernet block. If there are errors
during transmission of a multi-fragment frame the error statuses will be repeated until the
last fragment of the frame. 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. These may
include error information if the error is detected early enough. The status for the last
fragment in the frame will only be written after the transmission has completed on the
Ethernet connection. Thus, the status for the last fragment will always reflect any error
that occurred anywhere in the frame.
The status of the last frame transmission can also be inspected by reading the TSV0 and
TSV1 registers. These registers do not report statuses on a fragment basis and do not
store information of previously sent frames. They are provided primarily for debug
purposes, because the communication between driver software and the Ethernet block
takes place through the frame descriptors. The status registers are valid as long as the
internal status of the MAC is valid and should typically only be read when the transmit and
receive processes are halted.
Transmission error handling
If an error occurs during the transmit process, the Tx DMA manager will report the error
via the transmission StatusInfo word written in the Status array and the IntStatus interrupt
status register.
The transmission can generate several types of errors: LateCollision, ExcessiveCollision,
ExcessiveDefer, Underrun, and NoDescriptor. All have corresponding bits in the
transmission StatusInfo word. In addition to the separate bits in the StatusInfo word,
LateCollision, ExcessiveCollision, and ExcessiveDefer are ORed together into the Error
bit of the Status. Errors are also propagated to the IntStatus register; the TxError bit in the