Crossbar Switch (XBAR)MPC5566 Microcontroller Reference Manual, Rev. 27-10 Freescale SemiconductorThe only other time the XBAR has control of the slave port is when no masters are making access requeststo the slave port and the XBAR is forced to either park the slave port on a specific master, or place theslave port into low-power park mode. In these cases, the XBAR forces IDLE for the transfer type.7.3.5 Priority AssignmentEach master port must be assigned a unique 2-bit priority level in fixed priority mode. If multiple masterports are assigned the same priority level within a register (XBAR_MPR) undefined behavior results.7.3.6 ArbitrationXBAR supports two arbitration schemes; a simple fixed-priority comparison algorithm, and a round-robinfairness algorithm. The arbitration scheme is independently programmable for each slave port.7.3.6.1 Fixed Priority OperationWhen operating in fixed-priority arbitration mode, each master is assigned a unique priority level in theXBAR_MPR. If two masters both request access to a slave port, the master with the highest priority in theselected priority register gains control over the slave port.Any time a master makes a request to a slave port, the slave port checks to see if the new requestingmaster’s priority level is higher than that of the master that currently has control over the slave port (if any).The slave port does an arbitration check at every clock edge to ensure that the proper master (if any) hascontrol of the slave port.If the new requesting master’s priority level is higher than that of the master that currently has control ofthe slave port, the higher priority master is granted control at the termination of any currently pendingaccess, assuming the pending transfer is not part of a burst transfer.A new requesting master must wait until the end of the fixed-length burst transfer, before it is grantedcontrol of the slave port. But if the new requesting master’s priority level is lower than that of the masterthat currently has control of the slave port, the new requesting master is forced to wait until the master thatcurrently has control of the slave port is finished accessing the current slave port.7.3.6.2 Round-Robin Priority OperationWhen operating in round-robin mode, each master is assigned a relative priority based on the master portnumber. This relative priority is compared to the port number of the last master to perform a transfer onthe slave bus. The highest priority requesting master becomes the owner of the slave bus at the next transferboundary (accounting for fixed-length burst transfers). Priority is based on how far ahead the port numberof the requesting master is to the port number of the last master.After granted access to a slave port, a master may perform as many transfers as desired to that port untilanother master makes a request to the same slave port. The next master in line is granted access to the slaveport when the current transfer is completed, or possibly on the next clock cycle if the current master hasno pending access request.
