CS8900A
Crystal LAN™ ISA Ethernet Controller
It may become necessary for the host to change the
Destination Address (DA) filter criteria without re-
setting the CS8900A. This can be done as follows:
sponding bit in the 64-bit Logical Address Filter,
located at PacketPage 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 Ad-
dressFilteratPacketPagebase+0150h,ifnecessary.
Modify the Individual Address at PacketPage
base + 0158h, if necessary.
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, RxEvent, Bits A
through F).
3) Set SerRxON to re-enable the receiver.
5.3.3 Broadcast Frame Hashing Exception
Because the receiver has been disabled, the
CS8900A will ignore frames while the host is
changing the DA filter.
Table 25 describes in detail the content of the Rx-
Event register for each output of the hash and ad-
dress filters, and describes an exception to normal
processing. That exception can occur when when
5.3.2 Hash Filter
The hash filter is used to help determine which the hash-filter Broadcast address matches a bit in
Multicast frames and which Individual Address
frames should be accepted by the CS8900A.
the Logical Address Filter. To properly account for
this exception, the software driver should use the
following test to determine if the RxEvent register
contains a normal RxEvent (meaning bits E-A are
used for Extra data, Runt, CRC Error, Broadcast
and IndividualAdr) or a hash-table RxEvent (mean-
ing bits F-A contain the Hash Table Index).
5.3.2.1 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 register (HR).
The contents of the HR are passed through a 6-to-
64-bit decoder, asserting one of the decoder’s out-
puts. The asserted output is compared with a corre-
If bit Hashed =0, or bit RxOK=0, or (bits F-A = 02h
and the destination address is all ones) then Rx-
Event contains a normal RxEvent, else RxEvent
contained a hash RxEvent.
(MSB)
32-bit CRC value
(LSB)
Destination Address (DA)
from incoming frame
CS8900A
CRC
Logic
6-bit Hash Register (HR)
[Hash Table Index]
6-to-64 decoder
to
1
Hashed
bit
64
64-input
OR gate
64-bit Logical Address Filter (LAF)
Written into PacketPage base + 150h
Figure 23. Hash Filter Operation
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