Cirrus Logic CS8900A - Manual (Page 89)

DS271F5 89
CS8900A
Crystal LAN™ Ethernet Controller
CIRRUS LOGIC PRODUCT DATASHEET
5.2.13 Broadcast Frame Hashing Excep-
tion
Table 26 describes in detail the content of the
RxEvent register for each output of the hash
and address filters, and describes an excep-
tion to normal processing. That exception can
occur when the hash-filter Broadcast address
matches a bit in the Logical Address Filter. To
properly account for this exception, the soft-
ware driver should use the following test to de-
termine 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
(meaning bits F-A contain the Hash Table In-
dex).
If bit Hashed =0, or bit RxOK=0, or (bits F-A =
02h and the destination address is all ones)
then RxEvent 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
1
64
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
to
Hashed
bit
Figure 23. Hash Filter Operation
Address
Type of
Received
Frame
Erred
Frame?
Passes
Hash
Filter?
Contents of RxEvent
Bits F-A Bit 9
Hashed
Bit 8
RxOK
Bit 6
IAHash
Individual
Address
no yes Hash Table Index 1 1 1
no no ExtraData Runt CRC Error Broadcast Individual Adr 0 1 0
yes don’t care ExtraData Runt CRC Error Broadcast Individual Adr 0 0 0
Multicast
Address
no yes Hash table index 1 1 0
no no ExtraData Runt CRC Error Broadcast Individual Adr 0 1 0
yes don’t care ExtraData Runt CRC Error Broadcast Individual Adr 0 0 0
Notes: 6. Broadcast frames are accepted as Multicast frames if and only if all the following conditions are met
simultaneously:
a) the Logical Address Filter is programmed as: (MSB) 0000 8000 0000 0000h (LSB). Note that this
LAF value corresponds to a Multicast Addresses of both all 1s and 03-00-00-00-00-01.
b) the Rx Control Register (register 5) is programmed to accept IndividualA, MulticastA, RxOK-only,
and the following address filters were enabled: IAHashA and BroadcastA.
7. NOT (Note 1).
Table 26. Contents of RxEvent Upon Various Conditions