Freescale Semiconductor MPC860T Internal and External Loopback, Ethernet Error-Handling Procedure

Freescale Semiconductor, Inc.

Freescale Semiconductor, Inc.

3.10 Internal and External Loopback

The FEC supports Both internal and external loopback. In loopback mode, both FIFOs are used and the FEC operates in full-duplex fashion. Both internal and external loopback are conÞgured through R_CNTRL[LOOP, DRT].

For internal loopback, set LOOP = 1 and DRT = 0. TX_EN and TX_ER are not asserted during internal loopback.

For external loopback, set LOOP = 0 and DRT = 0. ConÞgure the external transceiver for loopback.

3.11 Ethernet Error-Handling Procedure

The FEC reports frame reception and transmission error conditions using the FEC BDs and the I_EVENT register.

3.11.1 Transmission Errors

Table 3-3 describes transmission errors.

	Table 3-3. Transmission Errors
Error	Description
Transmitter	If this error occurs, the FEC sends 32 bits that ensure a CRC error and stops transmitting. All
Underrun	remaining buffers for that frame are then ßushed and closed, with the UN bit set in the last TxBD for
	that frame. The FEC continues to the next TxBD and begins transmitting the next frame.
Carrier Sense	When this error occurs and no collision is detected in the frame, the FEC sets the CSL bit in the last
Lost during	TxBD for this frame. The frame is sent normally. No retries are performed as a result of this error.
Frame	The CSL bit is not set if X_CNTRL[FDEN] = 1, regardless of the state of CRS.
Transmission
Retransmission	When this error occurs, the FEC terminates transmission. All remaining buffers for that frame are
Attempts Limit	then ßushed and closed, with the RL bit set in the last TxBD for that frame. The FEC then continues
Expired	to the next TxBD and begins sending the next frame.
Late Collision	When this error occurs, the FEC stops sending. All remaining buffers for that frame are then ßushed
	and closed, with the LC bit set in the last TxBD for that frame. The FEC then continues to the next
	TxBD and begins sending the next frame.
	Note: The deÞnition of what constitutes a late collision is hard-wired in the FEC.
Heartbeat	Some transceivers have a self-test feature called heartbeat or signal-quality error. To signify a good
	self-test, the transceiver indicates a collision within 20 clocks after the FEC sends a frame. This
	heartbeat condition does not imply a real collision, but that the transceiver seems to work properly.
	If X_CNTRL[HBC] = 1, X_CNTRL[FDEN]=0, and a heartbeat condition is not detected after a frame
	transmission, a heartbeat error occursÑthe FEC closes the buffer, sets TxBD[HB], and generates
	the HBERR interrupt if it is enabled.

3.11.2 Reception Errors

Table 3-4 describes reception errors.

MOTOROLAChapter 3. Fast Ethernet Controller Operation3-7

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