Motorola MCF5282, MCF5281 user manual FEC Frame Reception

Functional Description

17.4.7 FEC Frame Reception

The FEC receiver is designed to work with almost no intervention from the host and can perform address recognition, CRC checking, short frame checking, and maximum frame length checking.

When the driver enables the FEC receiver by asserting ECR[ETHER_EN], it will immediately start processing receive frames. When ERXDV asserts, the receiver will first check for a valid PA/SFD header. If the PA/SFD is valid, it will be stripped and the frame will be processed by the receiver. If a valid PA/SFD is not found, the frame will be ignored.

In serial mode, the first 16 bit times of RX_D0 following assertion of ERXDV are ignored. Following the first 16 bit times the data sequence is checked for alternating 1/0s. If a 11 or 00 data sequence is detected during bit times 17 to 21, the remainder of the frame is ignored. After bit time 21, the data sequence is monitored for a valid SFD (11). If a 00 is detected, the frame is rejected. When a 11 is detected, the PA/SFD sequence is complete.

In MII mode, the receiver checks for at least one byte matching the SFD. Zero or more PA bytes may occur, but if a 00 bit sequence is detected prior to the SFD byte, the frame is ignored.

After the first 6 bytes of the frame have been received, the FEC performs address recognition on the frame.

Once a collision window (64 bytes) of data has been received and if address recognition has not rejected the frame, the receive FIFO is signalled that the frame is “accepted” and may be passed on to the DMA. If the frame is a runt (due to collision) or is rejected by address recognition, the receive FIFO is notified to “reject” the frame. Thus, no collision fragments are presented to the user except late collisions, which indicate serious LAN problems.

During reception, the Ethernet controller checks for various error conditions and once the entire frame is written into the FIFO, a 32-bit frame status word is written into the FIFO. This status word contains the M, BC, MC, LG, NO, CR, OV and TR status bits, and the frame length. See Section 17.4.14.2, “Reception Errors” for more details.

Receive Buffer (RXB) and Frame Interrupts (RFINT) may be generated if enabled by the EIMR register. A receive error interrupt is babbling receiver error (BABR). Receive frames are not truncated if they exceed the max frame length (MAX_FL); however, the BABR interrupt will occur and the LG bit in the Receive Buffer Descriptor (RxBD) will be set. See Section 17.6.2, “Ethernet Receive Buffer Descriptor (RxBD)” for more details.

When the receive frame is complete, the FEC sets the L-bit in the RxBD, writes the other frame status bits into the RxBD, and clears the E-bit. The Ethernet controller next generates a maskable interrupt (RFINT bit in EIR, maskable by RFIEN bit in EIMR), indicating that a frame has been received and is in memory. The Ethernet controller then waits for a new frame.

The Ethernet controller receives serial data LSB first.

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