Motorola MCF5281, MCF5282 user manual Ethernet Address Recognition

Functional Description

17.4.8 Ethernet Address Recognition

The FEC filters the received frames based on destination address (DA) type — individual (unicast), group (multicast), or broadcast (all-ones group address). The difference between an individual address and a group address is determined by the I/G bit in the destination address field. A flowchart for address recognition on received frames is illustrated in the figures below.

Address recognition is accomplished through the use of the receive block and microcode running on the microcontroller. The flowchart shown in Figure 17-2illustrates the address recognition decisions made by the receive block, while Figure 17-3illustrates the decisions made by the microcontroller.

If the DA is a broadcast address and broadcast reject (RCR[BC_REJ]) is deasserted, then the frame will be accepted unconditionally, as shown in Figure 17-2.Otherwise, if the DA is not a broadcast address, then the microcontroller runs the address recognition subroutine, as shown in Figure 17-3.

If the DA is a group (multicast) address and flow control is disabled, then the microcontroller will perform a group hash table lookup using the 64-entry hash table programmed in GAUR and GALR. If a hash match occurs, the receiver accepts the frame.

If flow control is enabled, the microcontroller will do an exact address match check between the DA and the designated PAUSE DA (01:80:C2:00:00:01). If the receive block determines that the received frame is a valid PAUSE frame, then the frame will be rejected. Note the receiver will detect a PAUSE frame with the DA field set to either the designated PAUSE DA or the unicast physical address.

If the DA is the individual (unicast) address, the microcontroller performs an individual exact match comparison between the DA and 48-bit physical address that the user programs in the PALR and PAUR registers. If an exact match occurs, the frame is accepted; otherwise, the microcontroller does an individual hash table lookup using the 64-entry hash table programmed in registers, IAUR and IALR. In the case of an individual hash match, the frame is accepted. Again, the receiver will accept or reject the frame based on PAUSE frame detection, shown in Figure 17-2.

If neither a hash match (group or individual), nor an exact match (group or individual) occur, then if promiscuous mode is enabled (RCR[PROM] = 1), then the frame will be accepted and the MISS bit in the receive buffer descriptor is set; otherwise, the frame will be rejected.

Similarly, if the DA is a broadcast address, broadcast reject (RCR[BC_REJ]) is asserted, and promiscuous mode is enabled, then the frame will be accepted and the MISS bit in the receive buffer descriptor is set; otherwise, the frame will be rejected.

In general, when a frame is rejected, it is flushed from the FIFO.

MOTOROLA Chapter 17. Fast Ethernet Controller (FEC) 17-11