PXD10RM Datasheet, PDF (279/1332 Pages) Freescale Semiconductor, Inc – PXD10 Microcontroller

This section examines data throughput from the point of view of masters and slaves, detailing when the
XBAR stalls masters, or inserts bubbles on the slave side.
10.6.2 General operation
When a master makes an access to the XBAR from an idle master state, the access is taken immediately
by the XBAR. If the targeted slave port of the access is available (that is, the requesting master is currently
granted ownership of the slave port), the access is immediately presented on the slave port. It is possible
to make single clock (zero wait state) accesses through the XBAR by a granted master. If the targeted slave
port of the access is busy or parked on a different master port, the requesting master receives wait states
until the targeted slave port can service the master request. The latency in servicing the request depends
on each master’s priority level and the responding slave’s access time.
Because the XBAR appears to be simply another slave to the master device, the master device has no
indication that it owns the slave port it is targeting. While the master does not have control of the slave port
it is targeting, it is wait-stated.
A master is given control of a targeted slave port only after a previous access to a different slave port has
completed, regardless of its priority on the newly targeted slave port. This prevents deadlock from
occurring when a master has the following conditions:
• Outstanding request to slave port A that has a long response time
• Pending access to a different slave port B
• Lower priority master also makes a request to the different slave port B.
In this case, the lower priority master is granted bus ownership of slave port B after a cycle of arbitration,
assuming the higher priority master slave port A access is not terminated.
After a master has control of the slave port it is targeting, the master remains in control of that slave port
until it 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 retains control of the slave port until that
transfer sequence is completed.
When a slave bus is idled by the XBAR, it is parked on the master which did the last transfer.
10.6.3 Master ports
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.
If the slave port is currently parked on another master, 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
Freescale Semiconductor
PXD10 Microcontroller Reference Manual, Rev. 1
Preliminary—Subject to Change Without Notice
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