PXN20RM Datasheet, PDF (436/1376 Pages) Freescale Semiconductor, Inc – PXN20 Microcontroller

AMBA Crossbar Switch (AXBS)
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 AXBS 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 AXBS.
16.3.4 Slave Ports
The goal of the AXBS with respect to the slave ports is to keep them 100% saturated when masters are
actively making requests. To do this the AXBS must not insert any bubbles onto the slave bus unless
absolutely necessary.
There is only one instance when the AXBS 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 AXBS has control of the slave port is when no masters are making access requests
to the slave port and the AXBS 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 AXBS forces IDLE for the transfer type.
16.3.5 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.
16.3.6 Arbitration
The AXBS 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.
16.3.6.1 Fixed Priority Operation
When operating in fixed-priority arbitration mode, each master is assigned a unique priority level in the
XBAR_MPR. If two masters both request access to a slave port, the master with the highest priority in the
selected 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 requesting
master’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) has
control of the slave port.
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PXN20 Microcontroller Reference Manual, Rev. 1
Freescale Semiconductor