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Req-1
Potential
Master 1
Potential
Master 2
Potential
Master n
Grant-1
Req-2
Central
Bus
Arbiter
Grant-n
Grant-2
Req-n
Bus Busy
Figure 8.13 Centralized arbiter with independent request and grant lines
bus requests from different levels, it grants the bus to the level with the highest
priority. Daisy chaining is used among the devices of that level. Figure 8.14
shows an example of four devices included in two priority levels. Potential
master 1 and potential master 3 are daisy-chained in level 1 and potential master
2 and potential master 4 are daisy-chained in level 2.
Decentralized Arbitration
In decentralized arbitration schemes, priority-based
arbitration is usually used in a distributed fashion. Each potential master has a
unique arbitration number, which is used in resolving conflicts when multiple
requests are submitted. For example, a conflict can always be resolved in favor of
the device with the highest arbitration number. The question now is how to deter-
mine which device has the highest arbitration number? One method is that a request-
ing device would make its unique arbitration number available to all other devices.
Each device compares that number with its own arbitration number. The device with
the smaller number is always dismissed. Eventually, the requester with the highest
arbitration number will survive and be granted bus access.
Grant level 1
Potential
Master 3
Potential
Master 1
Potential
Master 2
Potential
Master 4
Grant level 2
Central
Bus
Arbiter
Request level 1
Request level 2
Bus Busy
Figure 8.14 Centralized arbiter with two priority levels (four devices)
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