TNETX3150 datasheet(53/78 Pages) TI

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TNETX15AE

ADDRESS-LOOKUP DEVICE

SPWS041A – AUGUST 1997 – REVISED OCTOBER 1997

POST OFFICE BOX 655303

• DALLAS, TEXAS 75265

PRINCIPLES OF OPERATION

TNETX15AE operational modes

The TNETX15AE operational modes are selected through the control register. They are used to control decision

points in the logic operations. The modes available through the control register are NAUTO, NCRC, and NLRN0.

All three of these modes can be changed after the START bit is set high.

NAUTO mode

The not automatically add-address (NAUTO) mode is implemented to give the management CPU complete

control of the lookup table. It does so by disabling the two automatic processes that can affect the lookup table,

wire additions, and aging.

NAUTO to 1 disables adding addresses to the TNETX15VE tables learned from packets passing through the

TNETX3150/TNETX3150A. All addresses in the TNETX15VE must be added through the DIO interface (see

ADD in internal register section). No interrupt is generated on addresses added through the DIO interface.

NEW interrupts in this mode signal that the address-checking state machine has seen an address on the wire

that is not in the table.

NAUTO also affects the AGE logic operation by disabling it. AGE does not delete nodes from the table even

if the table is full. It is the management’s responsibility in this mode to maintain the addresses in the lookup table.

Table-full conditions are determined through a FULL interrupt, or when a host-initiated ADD stalls.

NCRC mode

The no-CRC (NCRC) check mode enables the TNETX15AE to add addresses learned from packets before the

CRC is checked. It is a performance-boosting feature for the ADD state machine that allows it to perform more

ADD logic operations in the same amount of time. This allows the TNETX15AE to add more efficiently and keep

the aging time stamp current on nodes that do not talk frequently on the network, avoiding unnecessary aging.

The tradeoff in this mode is the possibility that corrupt addresses could be added to the lookup table. This

condition should not become critical unless aging is turned off.

NRLN0 mode

When NRLN0 is set to 1, no new source address from a packet entering the switch on port 0 is added to the

table. This feature, combined with unkuniport and unkmultiport registers, allows the TNETX15AE to be at the

edge of a large network and only learn direct-attached station addresses, not the whole network above the

uplinked port.

aging modes (agingtimer register)

The TNETX15AE implements an autonomous aging logic operation. There are two modes of aging and these

modes are controlled through the agingtimer register. The two modes are time-threshold aging and

table-full aging.

Time-threshold aging is set by writing a value between 0x0001 and 0xFFFE to this register. In this mode, the

provides an aging range of 1 second to 18 hours. Addresses older than this are deleted.

Table-full aging is set by writing 0x0000 or 0xFFFF to the register. In this mode, the AGE logic operation ages

out addresses only when the table is full and the ADD logic operation needs to add an address. AGE deletes

the oldest address in the table. To identify the oldest address, the time stamp stored in each address descriptor

is incremented with each address added instead of each second of time.

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