gmacgrp_vlan_hash_table_reg

         
The 16-bit Hash table is used for group address filtering based on VLAN tag when Bit 18 (VTHM) of Register 7 (VLAN Tag Register) is set. For hash filtering, the content of the 16-bit VLAN tag or 12-bit VLAN ID (based on Bit 16 (ETV) of VLAN Tag 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 VLAN Hash table. For example, a hash value of 4b'1000 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 1'b1, the frame is accepted. Otherwise, it is rejected. Because the Hash Table register is double-synchronized to the (G)MII clock domain, the synchronization is triggered only when Bits[15:8] (in little-endian mode) of this register are written. 

Notes:
 * Because of double-synchronization, consecutive writes to this register should be performed after at least four clock cycles in the destination clock domain.