dp83261 National Semiconductor Corporation, dp83261 Datasheet - Page 57

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

6 0 Control Information
6 6 TIMER VALUES
The Ring Engine stores several timer values and thresholds used in normal operation With the exception of TNEG the timers
use an exponential expansion on a 4-bit value to produce a negative twos complement 24-bit value used by the Timer Logic
The timer values are always readable and are writable in Stop Mode
Asynchronous Priority Threshold (THSH1)
The Ring Engine currently supports one Asynchronous Priority Threshold (THSH1) in addition to the one at TTRT The Asyn-
chronous Priority Threshold is used in a magnitude comparison with THT when an Asynchronous Priority Request is presented
to the MAC Request Interface
Bits 7 – 4 are always written to Zero and are always read as Zero
When more than one threshold is used the users of THSH1 have the lowest priority All asynchronous transmissions are limited
by TTRT If the Late Flag is set no frames may be transmitted regardless of the value of the Asynchronous Priority Threshold
ACCESS RULES
THSH1
Address
87h
Zero
D7
Always
Read
Zero
D6
Warning The default value may not be appropriate for all values of TNEG
In some cases this could result in a request that is NEVER serviced
THSH1(3 – 0)
(Continued)
Stop Mode
Zero
A
B
C
D
E
F
0
1
2
3
4
5
6
7
8
9
D5
Write
Zero
D4
when token becomes unusable
Time remaining in THT
57
163 84 ms
327 68 ms
655 36 ms
1 3107 ms
2 6214 ms
5 2429 ms
10 486 ms
20 972 ms
41 943 ms (default)
83 886 ms
167 77 ms
335 54 ms
10 24 ms
20 48 ms
40 96 ms
81 92 ms
THSH(3)
D3
THSH(2)
D2
THSH(1)
D1
THSH(0)
D0