Freescale Semiconductor MPC5553, Reference Manual

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MPC5553/MPC5554 Microcontroller Reference Manual, Rev. 5
Freescale Semiconductor 7-9
slave port B. In this case, the lower priority master will be granted bus ownership of slave port B after a
cycle of arbitration, assuming the higher priority master’s slave port A access is not terminated.
After a master has control of the slave port it is targeting, the master will remain in control of that slave
port until it either gives up the slave port by running an IDLE cycle, leaves that slave port for its next
access, or loses control of the slave port to a higher priority master with a request to the same slave port.
However, because all masters run a fixed-length burst transfer to a slave port, it will retain control of the
slave port until that transfer sequence is completed. In round-robin arbitration mode, the current master
will be forced to hand off bus ownership to an alternately requesting master at the end of its current transfer
sequence.
When a slave bus is being idled by the XBAR, it can be parked on the master port indicated by the PARK
bits in the XBAR_SGPCR (slave general-purpose control register), or on the last master to have control
of the slave port. This can be done in an attempt to save the initial clock of arbitration delay that would
otherwise be seen if the master had to arbitrate to gain control of the slave port. The slave port can also be
put into low power park mode in attempt to save power.
7.3.3 Master Ports
The XBAR will terminate an access and it will not be 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 will be 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 will be completely transparent and the master’s access will be immediately
seen on the slave bus and no arbitration delays will be incurred. A master access will be stalled 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 will be 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 will gain 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 will gain 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 will be 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 will directly respond 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.
7.3.4 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. In order 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 will force 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