dp83261 National Semiconductor Corporation, dp83261 Datasheet - Page 69

• Manufacturer Part Number: dp83261
• Description: Bmac Device Fddi Media Access Controller
• Manufacturer: National Semiconductor Corporation

6 0 Control Information
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 ITC) 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
(TELR RLVLD) is set and may cause an interrupt When set RELR RLVLD indicates that RLCT will be valid within 1 28 ms The
Ring Latency Counter can measure ring latencies up to 1 3421772 seconds with accuracy of 1 28 ms
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
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
This counter is only writable while in STOP mode (Mode Run
RLCT0
RLCT1
RLCT2
RLCT3
Address
BC – BFh
CT(15)
CT(7)
Zero
Zero
D7
Always
Read
CT(14)
CT(6)
Zero
Zero
D6
(Continued)
Stop Mode
Write
CT(13)
CT(5)
Zero
Zero
D5
CT(12)
CT(4)
Zero
Zero
e
D4
69
0)
CT(19)
CT(11)
CT(3)
Zero
D3
CT(18)
CT(10)
CT(2)
Zero
D2
CT(17)
CT(9)
CT(1)
Zero
D1
CT(16)
CT(8)
CT(0)
Zero
D0
e
1)