DS3875 datasheet(8/58 Pages) NSC

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30 Introduction to DS3875

Arbitration Controller (Continued)

only describes one method of operation This method com-

pletely corresponds to the Unrestricted mode

In the

DS3875 the R-U bit in the CTRL2 register configures the

operation mode of the arbiter On reset the arbiter is placed

in the unrestricted mode in accordance to the 8961 specifi-

cation

Arbitration Competition Settling Time

Distributed Arbitration uses contention logic to select the

winner of the competition cycle The Arbitration competition

begins with all competing modules asserting the most signif-

icant bit AB 7

of the competition number onto the back-

plane lines A bit wise comparison is performed where mod-

ules are allowed to assert the subsequent bits only as long

as that module’s competition number bits thus far asserted

are greater than or equal to the bits seen on the backplane

lines Thus all competing modules begin asserting their

competition numbers onto the backplane lines but at the

end only the winning module will be allowed to assert it’s

competition number onto the backplane lines

When the DS3875 was designed two speeds where al-

lowed in the FBUSabackplane system for the arbitration

competition settling time Slow or fast During the arbitration

cycle modules would indicate if they had a fast or slow

competition settling time If any module indicated that it had

a slow competition settling time then all modules would

compete using the slow competition settling time else the

fast settling time would be used during the competition cy-

cle The DS3875 allows the user to program a fast or a slow

competition settling time One of eight settling times can be

selected for each mode with 16 total possible delay times

The IEEE 8961 specification now allows each module to

use it’s worst case arbitration settling time tA as the mod-

ule’s delay time to determine it’s winlose status for the

arbitration competition The ones and zeros combination in

the competition number plays a significant role in determin-

ing the numbers settling time Several factors are incorpo-

rated into the tA calculation

tpd

Maximum end to end signal propagation delay along

the transmission line (backplane) This is a function

of the maximum length of the transmission line

tint

Delay between a change on any arbitration line

ABx

on the

input to a module and the correspond-

ing

change on that module’s adjacent arbitration

output ab x-1

orabp

as its competition number

is applied to or withdrawn from those bits

text

Delay between a change on any arbitration line

ABx

onthe

input of any other competing module

and the corresponding

changes on that module’s ar-

bitration outputs ab x-1

orabp

as its competi-

tion number is applied to or withdrawn from those

bits

twin

Maximum delay between the time when the arbitra-

tion bus AB 70

becomes stable and equivalent

to cn 70 on the input to a module and the indica-

tion within that module that it is the winner

(Refer to 8961 for more details)

Thus now each module can evaluate it’s winlose condition

when it’s tA time has elapsed During a particular competi-

tion the winning module’s competition number will have the

longest worst case settling time since this is the module that

has placed all of it’s competition number bits onto the

FBUSa

To use the DS3875 in the 8961 compliant mode the user

programs two delay values in the CTRL2 register that are

closest to the calculated worst case settling time for the

particular competition number The F

S bit should corre-

spond to the table (fast or slow) that the most desired delay

resides The CTRL2 register bits PD3 to PD5 program the

delay number that corresponds to the fast mode operation

and PD0 to PD2 program the delay number that corre-

sponds to the slow mode operation Thus a delay that is

closest to the desired delay time should be selected both

from the fast (PD3 to PD5) table and the slow (PD0 to PD2)

table The F

S bit will configure this arbiter in the fast or

slow mode of operation

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