EMAC Registers
995
SLAU723A – October 2017 – Revised October 2018
Copyright © 2017–2018, Texas Instruments Incorporated
Ethernet Controller
15.6.39 EMACVLANHASH Register (Offset = 0x588) [reset = 0x0]
Ethernet MAC VLAN Hash Table (EMACVLANHASH)
The 16-bit hash table is used for group address filtering based on VLAN tag when the VTHM bit of the
EMACVLANTG register is set. For hash filtering, the content of the 16-bit VLAN tag or 12-bit VLAN ID
(enabled by ETV in the EMACVLANTG register) in the incoming frame is passed through the CRC logic
and the upper four bits of the calculated CRC are used to index the contents of the EMACVLANHASH
register. For example, a hash value of 0x8 selects bit 8 of the VLAN Hash table. The hash value of the
destination address is calculated in the following way:
1. Calculate the 32-bit CRC for the VLAN tag or ID (See IEEE 802.3, Section 3.2.8 for the steps to
calculate CRC32).
2. Perform bitwise reversal for the value obtained in Step 1.
3. Take the upper four bits from the value obtained in Step 2.
If the corresponding bit value of the register is 0x1, the frame is accepted. Otherwise, it is rejected.
EMACVLANHASH is shown in
and described in
.
Return to
Figure 15-54. EMACVLANHASH Register
31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10
9
8
7
6
5
4
3
2
1
0
RESERVED
VLHT
R-0x0
R/W-0x0
Table 15-63. EMACVLANHASH Register Field Descriptions
Bit
Field
Type
Reset
Description
31-16
RESERVED
R
0x0
15-0
VLHT
R/W
0x0
VLAN Hash Table. This field contains the 16-bit VLAN Hash Table.