FTCTO FTCT1

FTCT2 FTCT3

Always Stop Mode

D7 D6 D5 D4 D3 D2 D1 DO

Zero Zero Zero Zero Zero Zero Zero Zero

Zero Zero Zero Zero CT(19) CT(18) CT(17) CT(16)

CT(15) CT(14) CT(13) CT(12) CT(11) CT(10) CT(9) CT(8)

CT(7) CT(6) CT(5) CT(4) CT(3) CT(2) CT(1) CT(0)

2-196

Token Received Counter (TKCT)

The Token Received Counter (TKCT) maintains the count of valid tokens received by this station. The counter can be used with the Ring Latency Counter to calculate the average network load over a period of time. The frequency of token arrival is inversely related to the network load.

The Token Received Counter is incremented every time a valid token arrives.

Interrupts are available on increment (CILR.TKRCVD) and when the 20-bit counter overflows and wraps around (COLR.TKRCVD).

ACCESS RULES

6.0 Control Information

Address B8-BBh

Read Write

Always Stop Mode

D7 D6 D5 D4 D3 D2 D1 DO

TKCTO Zero Zero Zero Zero Zero Zero Zero Zero

TKCT1 Zero Zero Zero Zero CT(19) CT(18) CT(17) CT(16)

TKCT2 CT(15) CT(14) CT(13) CT(12) CT(11) CT(10) CT(9) CT(8)

TKCT3 CT(7) CT(6) CT(5) CT(4) CT(3) CT(2) CT(1) CT(0)

83 26 1

D P 83 26

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 (TELR.RLVD) 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 (TELR.RLVLD).

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 (Option.lTC) 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 6 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 (TELR.RLVLD) is set and may cause an interrupt. When set, RELR.RLVLD indicates that RLCT will be valid within 1.28 jus. The Ring Latency Counter can measure ring latencies up to 1.3421772 seconds with accuracy of 1.28 jlls.

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

Since a Master Reset (Function.MARST) causes TELR.RLVLD 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

6.0 Control Information

Address B C -B F h .

Read Always

Write Stop Mode

D7 D6 D5 D4 D3 D2 D1 DO

RLCTO Zero Zero Zero Zero Zero Zero Zero Zero

RLCT1 Zero Zero Zero 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 (Mode.Run= 1).

This counter is only writable while in STOP mode (Mode.Run = 0).

2-198

Late Count (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.

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.

Late Count 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 (Function.MCRST is set and Function.BCN and Function.CLM are not set) and every time the ring becomes non-operational. The Late Count Counter is a sticky counter at 15.

Events reported in the Token and Timer Event Latch Register (TELR.CBERR, TELR.TRTEXP) can be used to determine that Late Count Counter has incremented. No overflow event is provided.

ACCESS RULES

Address Read Write

OFh

6.0 Control Information

LTCT

Always n/a

D7 D6 D5 D4 D3 D2 D1 DO

Zero Zero Zero Zero CT3 CT2 CT1 CTO

83 26 1