DP83261AVF datasheet(69/132 Pages) NSC

69 / 132 page

60 Control Information (Continued)

Ring Latency Counter (RLCT)

The Ring Latency Counter (RLCT) is a measurement of time for PDUs to propagate around the ring This counter contains the

last measured ring latency whenever the RLVD bit of the Token and Timer Event Latch Register (TELRRLVD) is One

The current ring latency is measured by timing the propagation of a My

Void frame around the ring A new latency measure-

ment can be requested by clearing the Ring Latency Valid bit of the Token Event Register (TELRRLVLD)

When the ring is operational the next early token is captured Before the token is re-issued a My

Void frame is transmitted and

the Ring Latency Counter (RLCT) is reset The token will not be captured if the Inhibit Token Option (OptionITC) is set and the

ring latency will not be measured

When the ring is not operational ring latency timing will commence at the end of the next immediate request A My

Void is

transmitted and RLCT is reset This could be used to time how long the ring is non-operational since the My

Void frame will not

return

The Ring Latency Counter increments once every 16 byte times from when the Ending Delimiter of the My

Void frame is

transmitted until the Ending Delimiter of the My

Void frame returns When the My

Void frame returns the ring latency valid bit

(TELRRLVLD) is set and may cause an interrupt When set RELRRLVLD indicates that RLCT will be valid within 128 ms The

Ring Latency Counter can measure ring latencies up to 13421772 seconds with accuracy of 128 ms

The ring latency timing function is automatically disabled when exceptions are detected and retried at the next opportunity

Since a Master Reset (FunctionMARST) causes TELRRLVLD to be cleared the ring latency will automatically be measured on

the first opportunity (at the end of the first immediate request or with the first early token)

ACCESS RULES

Address

Read

Write

BC – BFh

Always

Stop Mode

RLCT0

Zero

RLCT1

Zero

CT(19)

CT(18)

CT(17)

CT(16)

RLCT2

CT(15)

CT(14)

CT(13)

CT(12)

CT(11)

CT(10)

CT(9)

CT(8)

RLCT3

CT(7)

CT(6)

CT(5)

CT(4)

CT(3)

CT(2)

CT(1)

CT(0)

In REV 1 of the BMAC device the Latency Counter is not reset to Zero when a new latency measurement is initiated The

latency count will be the difference between the value of RLCT after the measurement is complete and the value of RLCT

before the measurement was initiated

If a new latency measurement causes the latency counter to overflow the new latency value will be less than the previous value

In this case no subtraction is necessary The new value is equal to the ring latency This is because the Ring Engine recognizes

the overflow condition and restarts the latency count from zero

It is not possible to reset the Latency counter in software once the BMAC device has been put into RUN mode (ModeRune1)

This counter is only writable while in STOP mode (ModeRune0)

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