DP83821 datasheet(62/86 Pages) NSC

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4.0 Register Set (Continued)

4.2.22.3 Accept on Multicast or Unicast Hash

Multicast and Unicast addresses may be further qualified by use of the receive filter hash functions. An internal 2048 bit

(256 byte) RAM-based hash table is used to perform imperfect filtering of multicast or unicast packets. By enabling either

Multicast Hashing or Unicast Hashing in the RFCR, the receive filter logic will use the 11 most significant bits of the

destination addresses’ CRC as an index into the Hash Table memory. The upper 7 bits represent the word address and

the lower 4 bits select the bit within the word. If the corresponding bit is set, then the packet is accepted, otherwise the

packet is rejected. The hash table memory is accessed through the RFCR and the RFDR. Refer to Figure 4-2 for a

memory map. Below is example code for setting a bit in the hash table.

Figure 4-2 Hash Table Memory - 100h bytes addressed on word boundaries

Given a CRC of F9E80000:

RFCR = (IO base + 48h)

RFDR = (IO base + 4Ch)

# Bits 31-25 select which 16-bit word

Word = 7C

#Lower 24-21 bits select which bit in 16-bit word

Bit =F

# Select bit to set/clear

hash_bit = (0001<<bit)

#Write word address into RFCR

iowrite I RFCR (100 + word)

# Read indexed word from table

ioread I RFDR

set hash_word =(result | hash_bit) #OR in the hash bit to set

#Write to the hash table

iowrite I RFDR (hash_word)

# Enable multicast and/or unicast hash

iowrite I RFCR (RFEN | MHEN | UHEN)

X X

byte 255

byte 254

1FE

X X

byte 253

byte 252

1FC

X X

byte5

byte4

104

X X

byte3

byte2

102

X X

byte1

byte0

100

Bit#

17 16 15

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