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Table 18-29. How AXBS grants control of a slave port to a master (continued)
When Then AXBS grants control to the requesting master
The requesting master's priority level is lower than the current
master.
At the conclusion of one of the following cycles:
• An IDLE cycle
• A non-IDLE cycle to a location other than the current
slave port
18.3.3.3 Round-robin priority operation
When operating in Round-Robin mode, each master is assigned a relative priority based
on the master port number. This relative priority is compared to the master port number
(ID) of the last master to perform a transfer on the slave bus. The highest priority
requesting master becomes owner of the slave bus at the next transfer boundary,
accounting for locked and fixed-length burst transfers. Priority is based on how far ahead
the ID of the requesting master is to the ID of the last master.
After granted access to a slave port, a master may perform as many transfers as desired to
that port until another master makes a request to the same slave port. The next master in
line is granted access to the slave port at the next transfer boundary, or possibly on the
next clock cycle if the current master has no pending access request.
As an example of arbitration in Round-Robin mode, assume the crossbar is implemented
with master ports 0, 1, 4, and 5. If the last master of the slave port was master 1, and
master 0, 4 and 5 make simultaneous requests, they are serviced in the order 4, 5, and
then 0.
Parking may continue to be used in a round-robin mode, but does not affect the round-
robin pointer unless the parked master actually performs a transfer. Handoff occurs to the
next master in line after one cycle of arbitration. If the slave port is put into low-power
park mode, the round-robin pointer is reset to point at master port 0, giving it the highest
priority.
18.3.3.4 Priority assignment
Each master port must be assigned a unique 3-bit priority level. If an attempt is made to
program multiple master ports with the same priority level within the priority registers
(PRSn), the crossbar switch responds with a bus error and the registers are not updated.
Functional Description
K20 Sub-Family Reference Manual, Rev. 1.1, Dec 2012
360
Preliminary
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