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Fast Ethernet Controller (FEC)
Receive buffer (RXB) and frame interrupts (RFINT) may be generated if enabled by the EIMR register. A
receive error interrupt is a babbling receiver error (BABR). Receive frames are not truncated if they exceed
the max frame length (MAX_FL); however, the BABR interrupt occurs and the LG bit in the receive buffer
descriptor (RxBD) is set. See
Section 19.5.1.2, "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 is received and is in memory. The
Ethernet controller then waits for a new frame.
19.5.9
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 appears 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 19-27
illustrates the address recognition decisions made
by the receive block, while
Figure 19-28
illustrates the decisions made by the microcontroller.
If the DA is a broadcast address and broadcast reject (RCR[BC_REJ]) is cleared, then the frame is
accepted unconditionally, as shown in
Figure
19-27. Otherwise, if the DA is not a broadcast address, then
the microcontroller runs the address recognition subroutine, as shown in
Figure
19-28.
If the DA is a group (multicast) address and flow control is disabled, then the microcontroller performs 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 does an exact address match check between the DA and the
designated PAUSE DA (01:80:C2:00:00:01). If the receive block determines the received frame is a valid
PAUSE frame, the frame is rejected. The receiver detects a PAUSE frame with the DA field set to 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 you program 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 accepts or rejects the frame based on
PAUSE frame detection, shown in
Figure
19-27.
If neither a hash match (group or individual) nor an exact match (group or individual) occur, and if
promiscuous mode is enabled (RCR[PROM] set), the frame is accepted and the MISS bit in the receive
buffer descriptor is set; otherwise, the frame is rejected.
Similarly, if the DA is a broadcast address, broadcast reject (RCR[BC_REJ]) is asserted, and promiscuous
mode is enabled, the frame is accepted and the MISS bit in the receive buffer descriptor is set; otherwise,
the frame is rejected.
MCF5329 Reference Manual, Rev 3
Freescale Semiconductor
19-35
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