NXP Semiconductors UM10237

User manual Rev. 02 — 19 December 2008 254 o f 792

Chapter 11: LPC24XX Ethernet

IntStatus register is set in the case of a LateCollision, ExcessiveCollision, ExcessiveDefer,

or NoDescriptor error; Underrun errors are reported in the TxUnderrun bit of the IntStatus

Underrun errors can have three causes:

•The next fragment in a multi-fragment transmission is not available. This is a nonfatal

error. A NoDescriptor status will be returned on the previous fragment and the TxError

bit in IntStatus will be set.

•The transmission fragment data is not available when the Ethernet block has already

started sending the frame. This is a nonfatal error. An Underrun status will be returned

on transfer and the TxError bit in IntStatus will be set.

•The flow of transmission statuses stalls and a new status has to be written while a

previous status still waits to be transferred across the memory interface. This is a fatal

error which can only be resolved by a soft reset of the hardware.

The first and second situations are nonfatal and the device driver has to resend the frame

or have upper software layers resend the frame. In the third case the hardware is in an

undefined state and needs to be soft reset by setting the TxReset bit in the Command

After reporting a LateCollision, ExcessiveCollision, ExcessiveDefer or Underrun error, the

transmission of the erroneous frame will be aborted, remaining transmission data and

frame fragments will be discarded and transmission will continue with the next frame in

the descriptor array.

Device drivers should catch the transmission errors and take action.

Transmit triggers interrupts

The transmit datapath can generate four different interrupt types:

•If the Interrupt bit in the descriptor Control field is set, the Tx DMA will set the

TxDoneInt bit in the IntStatus register after sending the fragment and committing the

associated transmission status to memory. Even if a descriptor (fragment) is not the

last in a multi-fragment frame the Interrupt bit in the descriptor can be used to

generate an interrupt.

•If the descriptor array is empty while the Ethernet hardware is enabled the hardware

will set the TxFinishedInt bit of the IntStatus register.

•If the AHB interface does not consume the transmission statuses at a sufficiently high

bandwidth the transmission may underrun in which case the TxUnderrun bit will be set

in the IntStatus register. This is a fatal error which requires a soft reset of the

transmission queue.

•In the case of a transmission error (LateCollision, ExcessiveCollision, or

ExcessiveDefer) or a multi-fragment frame where the device driver did provide the

initial fragments but did not provide the rest of the fragments (NoDescriptor) or in the

case of a nonfatal overrun, the hardware will set the TxErrorInt bit of the IntStatus