MPC563XM Reference Manual, Rev. 1
Freescale Semiconductor
173
Preliminary—Subject to Change Without Notice
8.2.2
Features
The XBAR can support up to 8 master ports and 8 slave ports. If fewer master or slave ports are desired
the XBAR can be easily scaled down with the use of parameters and a wrapper. The XBAR can also be
configured in the same fashion to support either a 32-bit or 64-bit data bus width.
The XBAR has the ability to gain control of all the slave ports and prevent any masters from making
accesses to the slave ports. This feature is useful when the user wishes to turn off the clocks to the system
and needs to ensure that no bus activity will be interrupted.
The XBAR can put each slave port into a low power park mode so that slave port will not dissipate any
power transitioning address, control or data signals when not being actively accessed by a master port.
Each slave port can also support multiple master priority schemes. Each slave port has a hardware input
which selects the master priority scheme so the user can dynamically change master priority levels on a
slave port by slave port basis.
The XBAR will allow for concurrent transactions to occur from any master port to any slave port. It is
possible for all master ports and slave ports to be in use at the same time as a result of independent master
requests. If a slave port is simultaneously requested by more than one master port, arbitration logic will
select the higher priority master and grant it ownership of the slave port. All other masters requesting that
slave port will stalled until the higher priority master completes its transactions.
8.2.3
Limitations
The XBAR routes bus transactions initiated on the master ports to the appropriate slave ports. There is no
provision included to route transactions initiated on the slave ports to other slave ports or to master ports.
Simply put, the slave ports do not support the bus request/bus grant protocol, the XBAR assumes it is the
sole master of each slave port.
Since the XBAR does not support the bus request/bus grant protocol, if multiple masters are to be
connected to a single master port an external arbiter will need to be used. In the case of a single master
connecting to a master port the single master’s bus grant signal must be tied off in the asserted state.
Each master and slave port is fully AHB-Lite + AMBA V6 extensions compliant. The ports are not fully
AHB compliant because the XBAR does not support SPLITs or RETRYs.
8.2.4
General Operation
When a master makes an access to the XBAR the access will be immediately taken by the XBAR. If the
targeted slave port of the access is available then the access will be immediately presented on the slave
port. It is possible to make single clock (zero wait state) accesses through the XBAR. If the targeted slave
port of the access is busy or parked on a different master port the requesting master will simply see wait
states inserted (
hready
held negated) until the targeted slave port can service the master’s request. The
latency in servicing the request will depend on each master’s priority level and the responding peripheral’s
access time.
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