TNETX15AE datasheet(58/78 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

management address adding (continued)

The ADD logic operation locks the addnode, addVLANID, and addport registers to ensure that they do not

change during the address ADD. Reads to these registers are still possible, but writes are ignored. The ADD

bit in the adddelcontrol register remains a 1 until the ADD is complete.

Having a sticky bit (remains a 1) for ADD gives the programmer the opportunity to set up or perform other register

operations without having to wait for the ADD completion. A polling method is used to determine if the ADD

is finished.

adding unicast and multicast addresses

There is no significant change in procedure between adding unicast and multicast addresses. There is one

difference that the programmer must observe. The TNETX15AE stores different information for multicast

addresses than for unicast addresses. Unicast addresses use a 4-bit code for the port number and three flag

bits. Multicast addresses store a 15-bit forwarding mask.

Both data formats are added through the addport register. The format for this register changes, depending on

bit 40 in the addnode register. If set to 1, the addport data is interpreted as multicast (if not, it is interpreted

as unicast).

deleting an address

The TNETX15AE has two ways to delete addresses from the lookup table – the internal aging algorithm and

a host request through the DIO interface. The DEL state machine is responsible for deleting addresses from

the lookup table. DEL takes its information from the DIO registers for DIO deletes and from the AGE logic

operation for aging deletes.

The TNETX15AE implements a 16-bit timer, incrementing every second for the aging process. This timer is used

to write the time stamp during ADDs and for comparing ages.

aging (AGE logic operation)

The AGE logic operation is responsible for automatic address deletes. The TNETX15AE implements two styles

of aging: time-threshold aging and table-full aging. The aging style is selected through the agingtimer register.

A value of 0x0000 or 0xFFFF in the agingtimer register selects table-full aging. Any other value selects

time-threshold aging. The AGE logic operation is disabled when the TNETX15AE is placed in NAUTO mode.

The aging algorithm works as shown in Figure 16 and is described in the following.

Find Oldest

Start

Wait

for

Condition

Delete

Scan

Figure 16. Aging Algorithm Block Diagram

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