Shared Memory Interface
3-86 ADSP-21368 SHARC Processor Hardware Reference
Bus Arbitration Priority (RPBA)
To resolve competing bus requests, there are two available priority
schemes—fixed and rotating. The
RPBA pin selects the scheme. When RPBA
is high, rotating priority bus arbitration is selected, and when RPBA is low,
fixed priority is selected. The RPBA pin must be set to the same value on
each processor in a multiprocessing system.
In the fixed priority scheme, the processor with the lowest ID number
among the competing bus requests becomes the bus master. If, for exam-
ple, the processor with ID = 010 and the processor with ID =100 request
the bus simultaneously, the processor with ID=010 becomes bus master in
the following cycle.
L
Each processor knows the ID of the other processors requesting the
bus, because the ID corresponds to the BRx line being used for each
processor.
The rotating priority scheme gives roughly equal priority to each proces-
sor. When rotating priority is selected, the priority of each processor is
reassigned after every transfer of bus mastership. Highest priority is
rotated from processor to processor as if they were arranged in a circle—
the processor with the next highest ID setting from the current bus master
is the one that receives highest priority. Table 3-31 shows an example of
how rotating priority changes on a cycle-by-cycle basis.
Table 3-31. Rotating Priority Arbitration Example
Cycle Number
Hardwired Processor IDs and Priority
1
ID1 ID2 ID3 ID4
1
2
M 1 2-BR 3
2 2 3-BR M-BR 1
3 2 3-BR M1
To Next Page
Loading...
Need help?
General
