Cirrus Logic CS8900A - Manual (Page 88)

88 DS271F5
CS8900A
Crystal LAN™ Ethernet Controller
CIRRUS LOGIC PRODUCT DATASHEET
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.
It may become necessary for the host to
change the Destination Address (DA) filter cri-
teria without resetting the CS8900A. This can
be done as follows:
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.
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.
5.2.12.1 Hash Filter Operation
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 Packet-
Page 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.
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).
IAHashA PromiscuousA MulticastA IndividualA BroadcastA Frames Accepted
0 0 0 1 0 Individual Address frames with
DA matching the IA at Packet-
Page base + 0158h
1 0 0 0 0 Individual Address frames with
DA that pass the hash filter
(DA[0] must be “0”)
0 0 1 0 0 Multicast frames with DA that
pass the hash filter (DA[0] must
be “1”)
0 0 0 0 1 Broadcast frames
X 1 X X X All frames
Table 25. DA Filtering Options