Functional Description

17.4.14.1.4 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 to the FEC within 4 microseconds after completion of a frame transmitted by the Ethernet controller. This indication of a collision does not imply a real collision error on the network, but is rather an indication that the transceiver still seems to be functioning properly. This is called the heartbeat condition.

If the HBC bit is set in the TCR register and the heartbeat condition is not detected by the FEC after a frame transmission, then a heartbeat error occurs. When this error occurs, the FEC closes the buffer, sets the HB bit in the EIR register, and generates the HBERR interrupt if it is enabled.

17.4.14.2 Reception Errors

17.4.14.2.1 Overrun Error

If the receive block has data to put into the receive FIFO and the receive FIFO is full, the FEC sets the OV bit in the RxBD. All subsequent data in the frame will be discarded and subsequent frames may also be discarded until the receive FIFO is serviced by the DMA and space is made available. At this point the receive frame/status word is written into the FIFO with the OV bit set. This frame must be discarded by the driver.

17.4.14.2.2 Non-Octet Error (Dribbling Bits)

The Ethernet controller handles up to seven dribbling bits when the receive frame terminates past an non-octet aligned boundary. Dribbling bits are not used in the CRC calculation. If there is a CRC error, then the frame non-octet aligned (NO) error is reported in the RxBD. If there is no CRC error, then no error is reported.

17.4.14.2.3 CRC Error

When a CRC error occurs with no dribble bits, the FEC closes the buffer and sets the CR bit in the RxBD. CRC checking cannot be disabled, but the CRC error can be ignored if checking is not required.

17.4.14.2.4 Frame Length Violation

When the receive frame length exceeds MAX_FL bytes the BABR interrupt will be generated, and the LG bit in the end of frame RxBD will be set. The frame is not truncated unless the frame length exceeds 2047 bytes).

MOTOROLA

Chapter 17. Fast Ethernet Controller (FEC)

17-19

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Motorola MCF5281, MCF5282 user manual Reception Errors, Heartbeat