PXR40RM Datasheet, PDF (442/1434 Pages) Freescale Semiconductor, Inc – PXR40 Microcontroller

AMBA Crossbar Switch (XBAR)
14.3.3 Master Ports
The XBAR terminates an access and it is not allowed to pass through the XBAR unless the master
currently is granted access to the slave port to which the access is targeted. A master access is taken if the
slave port to which the access decodes is either currently servicing the master or is parked on the master.
In this case, the XBAR is completely transparent and the master access is immediately transmitted on the
slave bus and no arbitration delays are incurred. A master access stall if the access decodes to a slave port
that is busy serving another master, parked on another master or is in low-power park mode.
If the slave port is currently parked on another master or is in low-power park mode, and no other master
is requesting access to the slave port, then only one clock of arbitration is incurred. If the slave port is
currently serving another master of a lower priority and the master has a higher priority than all other
requesting masters, then the master gains control over the slave port as soon as the data phase of the current
access is completed. If the slave port is currently servicing another master of a higher priority, then the
master gains control of the slave port after the other master releases control of the slave port if no other
higher priority master is also waiting for the slave port.
A master access is responded to with an error if the access decodes to a location not occupied by a slave
port. This is the only time the XBAR directly responds with an error response. All other error responses
received by the master are the result of error responses on the slave ports being passed through the XBAR.
14.3.4 Slave Ports
The goal of the XBAR with respect to the slave ports is to keep them 100% saturated when masters are
actively making requests. To do this the XBAR must not insert any bubbles onto the slave bus unless
absolutely necessary.
There is only one instance when the XBAR forces a bubble onto the slave bus when a master is actively
making a request. This occurs when a handoff of bus ownership occurs and there are no wait states from
the slave port. A requesting master which does not own the slave port is granted access after a one clock
delay.
The only other time the XBAR has control of the slave port is when no masters are making access requests
to the slave port and the XBAR is forced to either park the slave port on a specific master, or place the
slave port into low-power park mode. In these cases, the XBAR forces IDLE for the transfer type.
14.3.5 Priority Assignment
Each master port must be assigned a unique 2-bit priority level in fixed priority mode. If multiple master
ports are assigned the same priority level within a register (XBAR_MPR) undefined behavior results.
14.3.6 Arbitration
XBAR supports two arbitration schemes; a simple fixed-priority comparison algorithm, and a round-robin
fairness algorithm. The arbitration scheme is independently programmable for each slave port.
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PXR40 Microcontroller Reference Manual, Rev. 1
Freescale Semiconductor