DS80C400 Maxim, DS80C400 Datasheet - Page 63

• Manufacturer Part Number: DS80C400
• Description: The DS80C400 network microcontroller offers the highest integration available in an 8051 device
• Manufacturer: Maxim

Multicast Hash Filter
Hash filtering of destination addresses requires that a hash table be established. The hash table must be
programmed into the CSR multicast address low and multicast address high registers. When hash filtering has
been selected, the 6 bytes of the destination address are passed through the internal CRC-32 logic. The most
significant 6 bits of the resultant CRC-32 are used to index into the hash table. From those 6 bits, the most
significant bit determines whether multicast address high or low is used, while the lower 5 bits provide the bit index
into the selected CSR register. If the multicast address high or low register bit indexed by the upper 6 bits of the
CRC-32 is programmed to 1, then the destination address passes.
Figure 8. Hash Table Index Generation
VLAN Support
The DS80C400 offers VLAN support through recognition of frames that are tagged as such. Each VLAN tag
provides tag control information (TCI) containing a tag protocol ID (TPID) and VLAN ID. The incoming TPID occupy
the 13th and 14th byte positions, those that would normally contain either the length or type field for the frame. The
TPID is compared against the VLAN1 (20h) and VLAN2 (24h) CSR registers.
If a non-zero match occurs between the TPID and VLAN1 register setting, the frame is recognized as having a
VLAN1 tag. For VLAN1 tagged frames, the TPID is followed by 2 Bytes containing the VLAN ID; therefore, the
MAC extends the maximum legal frame length by a total of 4 Bytes (TPID = 2 Bytes, VLAN ID = 2 Bytes).
If a non-zero match occurs between the TPID and VLAN2 register setting, the frame is recognized as having a
VLAN2 tag. For VLAN2 tagged frames, the TPID is followed by 18 Bytes containing the VLAN ID; therefore, the
MAC extends the maximum legal frame length by a total of 20 Bytes (TPID = 2 Bytes, VLAN ID = 18 Bytes).
Figure 9. VLAN Tagged Frame
PREAMBLE SFD
PREAMBLE SFD DESTINATION ADDRESS
(7)
(1)
DESTINATION ADDRESS
(6)
CRC-32 GENERATOR
POLYNOMIAL
SOURCE ADDRESS
SOURCE ADDRESS TYPE/LENGTH
ETHERNET FRAME
(6)
TPID
(2)
TCI
63 of 97
ETHERNET FRAME
TYPE/LENGTH
(18: VLAN2)
(2: VLAN1)
VLAN ID
(2)
CRC-32 OF DESTINATION ADDRESS BYTES
Figure 8
shows the hash filtering process.
(46-1500)
BIT INDEX
MULTICAST REGISTER SELECT
DATA
DATA
00000b: bit 0
00001b: bit 1
11110b: bit 30
11111b: bit 31
0: Multicast Address Low
1: Multicast Address High
. . .
.
.
. . .
CRC-32
(4)
CRC-32