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Table 9-1. How the Crossbar Switch grants control of a slave port to a master
When
Then the Crossbar Switch grants control to the
requesting master
Both of the following are true:
• The current master is not running a transfer.
• The new requesting master's priority level is higher than
that of the current master.
At the next clock edge
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
9.4.2.3 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. 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 then 5
then 0.
The round-robin arbitration mode generally provides a more fair allocation of the
available slave-port bandwidth (compared to fixed priority) as the fixed master priority
does not affect the master selection.
9.5 Initialization/application information
No initialization is required for the crossbar switch. See the chip-specific crossbar switch
information for the reset state of the arbitration scheme.
Chapter 9 Crossbar Switch Lite (AXBS-Lite)
Kinetis KE1xZ256 Sub-Family Reference Manual, Rev. 3, 07/2018
NXP Semiconductors
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