DP83266 datasheet(85/152 Pages) NSC

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70 Control Information (Continued)

Late Count Counter (LTCT)

The Late Count Counter (LTCT) is implemented differently than suggested by the FDDI MAC Standard but provides similar

information The function of the Late Count Counter is divided between the Late

Flag and a separate counter The Late

Flag

is equivalent to the Standard Late Count with a non-zero value It is maintained by the Ring Engine to indicate if it is possible to

send asynchronous traffic When the ring is operational Late Count indicates the time it took the ring to recover the last time the

ring went non-operational When the ring is non-operational Late Count indicates the time it has taken (so far) to recover the

ring

The Late Count is provided to assist Station Management in the isolation of serious ring errors In many situations it is helpful for

SMT to know how long it has been since the ring went non-operational in order to determine if it is necessary to invoke recovery

procedures When the ring becomes non-operational there is no way to know how long it will stay non-operational therefore a

timer is necessary If the Late Count Counter is not provided SMT would be forced to start a timer every time the ring goes non-

operational even though it may seldom be used By using the provided Late Count Counter an SMT implementation may be able

to alleviate this additional overhead

The Late Count Counter is incremented every time TRT expires while the ring is non-operational and Late

Flag is set (once

every TMAX) This counter is never writable not even in Stop Mode The counter is set to Zero as a result of a MAC Reset when

a Beacon or Claim Request is not also present (FunctionMCRST is set and FunctionBCN and FunctionCLM are not set) and

every time the ring becomes non-operational Late Count Counter is a sticky counter at 15

Events reported in the Token and Timer Event Latch Register (TELR0CBERR TELR0TRTEXP) can be used to determine that

Late Count Counter has incremented No overflow event is provided

Access Rules

Address

Read

Write

09Fh

Always

LTCT

Zero

CT3

CT2

CT1

CT0

DP83266 Datasheet(PDF) 85 Page - National Semiconductor (TI)