DS3875 datasheet(17/58 Pages) NSC

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50 Arbitrating for Futurebusa (Continued)

TLH10747 – 10

d Message Receiving

TLH10747 – 11

e Message Sending

FIGURE 5 Bus Request Timing (Normal Arbitration Idle Bus Arbitration and Parking)

(Continued)

54 FUTUREBUSa OPTIONAL MEANS OF

ARBITRATION

Besides the normal Futurebusa Parallel Contention Arbitra-

tion the Futurebusa Specification allows two other option-

al arbitration methods to acquire the parallel bus quickly

Idle Bus Arbitration

Parking

Both of these arbitration methods are supported by the Na-

tional Semiconductor DS3875 Arbitration Controller

541 Idle Bus Arbitration (IBA)

Idle Bus Arbitration (IBA) is selected by setting the IBAPK

bit in CTRL3 register The aim of IBA is to give a module

quick access to Futurebusa when the current master has

completed its transfers

IBA is invoked the same as a normal arbitration bus request

( BRQ ) but uses the parallel bus (AS negated DS as-

serted and the parallel addressdata bus) to determine the

winner of the arbitration cycle see

Figure 5a b

During normal arbitration the transfer of Futurebusa tenure

occurs during phase 5 During IBA the transfer of Future-

busa tenure occurs during phase 2 During IBA the AC1O

arbitration condition signal will be asserted by the new mas-

ter This will cancel the transfer of tenure during the normal

arbitration cycle Note that Futurebusa tenure was trans-

ferred but only the two modules involved in the IBA (the

master and the master elect) know that the transfer of ten-

ure has taken place All the other modules involved in the

normal arbitration cycle see that the transfer of tenure dur-

ing the normal arbitration cycle was canceled

If two or more modules have a request then normal arbitra-

tion will determine which module will gain access IBA takes

place on the parallel highway ADDATA (31

0) Each

module is assigned a bit which is to be asserted during IBA

If only one bit is asserted during the IBA competition then

IBA issues the bus grant signal to that module Concurrently

with IBA normal arbitration takes place If a module does

not want IBA to issue a bus grant IBA may be inhibited by

asserting DI

During phase 5 normal arbitration will deter-

mine appropriate actions

5411 Masters Support Circuitry to Enable IBA

If IBA is allowed it is invoked during phase 0 In order to

support IBA external logic is needed in addition to the arbi-

tration controller The Masters external logic must monitor

the arbitration control acquisition synchronization signals

(AP

AR ) and the parallel bus address handshake

signals (AS

AI ) If the master supports IBA it should

wait until it is ready to end its bus tenure (ENDT asserted)

Then when it releases AS it should assert ds to enable

modules to participate in IBA competition

5412 Modules Support Circuitry to Participate in IBA

If a module wants to use IBA to gain tenure of Futurebusa

it must use external logic to monitor the arbitration control

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