CS8900A
Crystal LAN™ Ethernet Controller
The IAHashA, MulticastA, IndividualA, and
BroadcastA bits are used independently. As a
result, many DA filter combinations are possi-
ble. For example, if MulticastA and IndividualA
are set, then all frames that are either Multicast
or Individual Address frames are accepted.
The PromiscuousA bit, when set, overrides the
other four DA bits, and allows all valid frames
to be accepted. Table 25 summarizes the con-
figuration options available for DA filtering.
IAHashA
PromiscuousA MulticastA IndividualA
BroadcastA
Frames Accepted
0
0
0
0
0
0
1
1
0
0
0
Individual Address frames with
DA matching the IA at Pack-
etPage base + 0158h
1
0
0
0
Individual Address frames with
DA that pass the hash filter
(DA[0] must be “0”)
Multicast frames with DA that
pass the hash filter (DA[0] must
be “1”)
0
0
1
0
0
1
Broadcast frames
All frames
X
X
X
X
Table 25. DA Filtering Options
It may become necessary for the host to 5.2.12.1 Hash Filter Operation
change the Destination Address (DA) filter cri-
teria without resetting the CS8900A. This can
be done as follows:
See Figure 23. The DA of the incoming frame
is passed through the CRC logic, generating a
32-bit CRC value. The six most-significant bits
of the CRC are latched into the 6-bit hash reg-
ister (HR). The contents of the HR are passed
through a 6-to-64-bit decoder, asserting one of
the decoder's outputs. The asserted output is
compared with a corresponding bit in the 64-
bit Logical Address Filter, located at Pack-
etPage base + 0150h. If the decoder output
and the Logical Address Filter bit match, the
frame passes the hash filter and the Hashed
bit (Register 4, RxEvent, Bit 9) is set. If the two
do not match, the frame fails the filter and the
Hashed bit is clear.
1) Clear SerRxON (Register 13, LineCTL, Bit
6) to prevent any additional receive frames
while the filter is being changed.
2) Modify the DA filter bits (B, A, 9, 7, and 6)
in the RxCTL register. Modify the Logical
Address Filter at PacketPage base +
0150h, if necessary. Modify the Individual
Address at PacketPage base + 0158h, if
necessary.
3) Set SerRxON to re-enable the receiver.
Because the receiver has been disabled, the
CS8900A will ignore frames while the host is
changing the DA filter.
Whenever the hash filter is passed by a "good"
frame, the RxOK bit (Register 4, RxEvent, Bit
8) is set and the bits in the HR are mapped to
the Hash Table Index bits (Register 4, Rx-
Event, Bits A through F).
5.2.12 Hash Filter
The hash filter is used to help determine which
Multicast frames and which Individual Address
frames should be accepted by the CS8900A.
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