TNETX15VE datasheet(77/113 Pages) TI1

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TNETX15VE

VLAN-ENGINE ADDRESS-LOOKUP DEVICE

SPWS028B – APRIL 1997 – REVISED SEPTEMBER 1997

POST OFFICE BOX 655303

• DALLAS, TEXAS 75265

PRINCIPLES OF OPERATION

NLRN mode

The no-learn (NLRN) mode disables the automatic wire learning of node addresses that are assigned to a port

on a port-by-port basis. While learning is turned off, no inbound frames can affect time stamps for addresses

already received. If the address was present in the table and aging is running, the address can age out even

if frames are being received from that source address. NLRN mode does not disable a port and prevent it from

receiving or transmitting frames.

TX block mode

The transmit-blocking (TX block) mode disables the transmission of frames by any port in this mode. Frames

are still received and forwarded to other ports from this port with this mode set.

RX block mode

The receive-blocking (RX block) mode blocks all frames from being received by any port in this mode. Address

learning from this port is not disabled in this mode.

TX block, RX block and NLRN can be used in combinations.

lookup table SRAM initialization (START)

The lookup table is automatically initialized by the TNETX15VE with no need for an external processor. The

steps for initializing are:

• Write to RAMsize the size of the external SRAM. Writing to RAMsize can be performed by a CPU or the

EEPROM auto-load operation.

• Assert the START bit in the control register. This is accomplished either by the CPU or EEPROM.

• TheTNETX15VEindicatesthecompletionofthelookup-tableinitializationbysettingtheINITDbitinthe

control register to 1.

TNETX15VE clears the lookup table by writing 0x0000 to all available locations and then initializes the data

structures (FIFO, lookup tables). After these operations are done, The TNETX15VE performs lookups, adding,

and aging operations as directed by mode bits.

frame forwarding-LKUP logic operation

The lookup (LKUP) logic operation is designed for two important tasks: 1) perform time-critical lookups of the

wire within the TNETX3150/TNETX3150A’s allotted time, and 2) forward the frame to the right ports. The LKUP

logic operation works independently from all other logic operations and from the management CPU. To meet

the timing requirements, this logic operation occupies the highest priority on the SRAM bus.

LKUP logic operation performs a lookup on the destination address of the frame, plus a 2-bit VLAN, and routes

traffic accordingly. The destination address is found from the DRAM interface. The VLANID used is determined

from the tag field and the source port, as described previously.

The lookup logic operation routes the frame differently, depending on whether the frame is found in the lookup

table. Other registers and conditions also affect these routing options. The registers that affect routing options

are VLANID, VLANmask, tagVLAN, control, TXblock, RXblock, mirrorport, and uplinkport. The LOCKED and

CUPLNK bits contained in the lookup table also affect the routing options for unicasts. The NBLCK flag affects

the routing options for unicasts and multicasts.

Nominally, if an address is found, the TNETX15VE uses the information contained in the lookup table for routing.

If the address is not found, the TNETX15VE broadcasts the frame to the ports indicated in the VLANmask

the packet to additional ports.

TNETX15VE 数据表(PDF) 77 Page - Texas Instruments

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