VLAN_Tag

The VLAN Tag register contains the IEEE 802.1Q VLAN Tag to identify the VLAN frames. The MAC compares the 13th and 14th bytes of the receiving frame (Length/Type) with 16'h8100, and the following two bytes are compared with the VLAN tag. If a match occurs, the MAC sets the received VLAN bit in the receive frame status. The legal length of the frame is increased from 1,518 bytes to 1,522 bytes. Because the VLAN Tag register is double-synchronized to the (G)MII clock domain, then consecutive writes to these register should be performed only after at least four clock cycles in the destination clock domain.

Module Instance	Base Address	Register Address
`emac0`	`0xFF700000`	`0xFF70001C`
`emac1`	`0xFF702000`	`0xFF70201C`

Offset: 0x1C

Access: RW

Important: To prevent indeterminate system behavior, reserved areas of memory must not be accessed by software or hardware. Any area of the memory map that is not explicitly defined as a register space or accessible memory is considered reserved.

Bit Fields
31	30	29	28	27	26	25	24	23	22	21	20	19	18	17	16
`Reserved`												`vthm` `RW 0x0`	`esvl` `RW 0x0`	`vtim` `RW 0x0`	`etv` `RW 0x0`
15	14	13	12	11	10	9	8	7	6	5	4	3	2	1	0
`vl` `RW 0x0`

VLAN_Tag Fields

Bit

Name

Description

Access

Reset

vthm

When set, the most significant four bits of the VLAN tag's CRC are used to index the content of Register 354 (VLAN Hash Table Register). A value of 1 in the VLAN Hash Table register, corresponding to the index, indicates that the frame matched the VLAN hash table. When Bit 16 (ETV) is set, the CRC of the 12-bit VLAN Identifier (VID) is used for comparison whereas when ETV is reset, the CRC of the 16-bit VLAN tag is used for comparison. When reset, the VLAN Hash Match operation is not performed.

RW

0x0

esvl

When this bit is set, the MAC transmitter and receiver also consider the S-VLAN (Type = 0x88A8) frames as valid VLAN tagged frames.

RW

0x0

vtim

When set, this bit enables the VLAN Tag inverse matching. The frames that do not have matching VLAN Tag are marked as matched. When reset, this bit enables the VLAN Tag perfect matching. The frames with matched VLAN Tag are marked as matched.

RW

0x0

etv

When this bit is set, a 12-bit VLAN identifier is used for comparing and filtering instead of the complete 16-bit VLAN tag. Bits [11:0] of VLAN tag are compared with the corresponding field in the received VLAN-tagged frame. Similarly, when enabled, only 12 bits of the VLAN tag in the received frame are used for hash-based VLAN filtering. When this bit is reset, all 16 bits of the 15th and 16th bytes of the received VLAN frame are used for comparison and VLAN hash filtering.

Value	Description
0x0	Disable 12-Bit VLAN Tag Compare
0x1	Enable 12-Bit VLAN Tag Compare

RW

0x0

15:0

vl

This field contains the 802.1Q VLAN tag to identify the VLAN frames and is compared to the 15th and 16th bytes of the frames being received for VLAN frames. The following list describes the bits of this field: * Bits [15:13]: User Priority * Bit 12: Canonical Format Indicator (CFI) or Drop Eligible Indicator (DEI) * Bits[11:0]: VLAN tag's VLAN Identifier (VID) field When the ETV bit is set, only the VID (Bits[11:0]) is used for comparison. If VL (VL[11:0] if ETV is set) is all zeros, the MAC does not check the fifteenth and 16th bytes for VLAN tag comparison, and declares all frames with a Type field value of 0x8100 or 0x88a8 as VLAN frames.

RW

0x0