CS8900A datasheet(89/128 Pages) CIRRUS

89 / 128 page

DS271PP3

CS8900A

Crystal LAN™ ISA Ethernet Controller

CIRRUS LOGIC PRODUCT DATA SHEET

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:

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.

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.

5.3.2 Hash Filter

The hash filter is used to help determine which

Multicast frames and which Individual Address

frames should be accepted by the CS8900A.

5.3.2.1 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-

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.

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).

5.3.3 Broadcast Frame Hashing Exception

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

the hash-filter Broadcast address matches a bit in

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).

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.

64-bit Logical Address Filter (LAF)

Written into PacketPage base + 150h

6-to-64 decoder

CS8900A

CRC

Logic

Destination Address (DA)

from incoming frame

32-bit CRC value

(MSB)

(LSB)

6-bit Hash Register (HR)

[Hash Table Index]

64-input

OR gate

Hashed

bit

Figure 23. Hash Filter Operation

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