LH79524/LH79525 User’s Guide Ethernet MAC Controller
Version 1.0 6-13
6.1.5.4 Type ID Checking
The contents of the IDCHK register are compared against the Length/Type ID of
received frames. Bit 22 in the Receive Buffer Descriptor List is 1 if there is a match
(see Table 6-1). The reset state of IDCHK is zero, which is unlikely to match the
Length/Type ID of any valid Ethernet frame.
6.1.5.5 VLAN Support
An Ethernet encoded 802.1Q VLAN tag contains the parameters in Table 6-4.
The VLAN tag is inserted at the 13th byte of the frame, adding four bytes to the frame
length. If the VID (VLAN identifier) is null (0x000), this indicates a priority-tagged frame.
The MAC can support frame lengths up to 1,536 bytes, 18 bytes more than the original
Ethernet maximum frame length of 1,518 bytes. This is enabled by programming the
NETCONFIG:RECBYTE bit to 1.
Bits [21:16] of the Receive Buffer Descriptor List (see Table 6-1) provide information about
VLAN tagged frames:
• Bit 21= 1 if the receive frame is VLAN tagged (Type ID = 0x8100)
• Bit 20 = 1 if receive frame is priority tagged (Type ID = 0x8100 and null VID); bit 21 must
also = 1
• Bits 19, 18, and 17 = 1 if bit 21 is set
• Bit 16 = 1 if bit 21 is set
6.2 Programming Model
This section provides a programming model and example for the EMAC. Also, refer to
Section 6.3, Register Reference, for specific register and bit definitions.
Table 6-4. VLAN Support
PARAMETER DEFINITION
TPID (Tag Protocol Identifier) 16 bits TCI (Tag Control Information) 16 bits
0x8100 First 3 bits priority, then CFI bit, last 12 bits VID
To Next Page
Loading...