mcf51jf128 Freescale Semiconductor, Inc, mcf51jf128 Datasheet - Page 326

• Manufacturer Part Number: mcf51jf128
• Description: Mcf51jf128 Reference Manual
• Manufacturer: Freescale Semiconductor, Inc

Functional Description
14.3 Functional Description
This section provides the functional details of the crossbar switch.
14.3.1 General Operation
When a master accesses the crossbar switch the access is immediately taken. If the
targeted slave port of the access is available then the access is immediately presented on
the slave port. It is possible to make single-clock (zero wait state) accesses through the
crossbar. If the targeted slave port of the access is busy or parked on a different master
port, the requesting master simply sees wait states inserted until the targeted slave port
can service the master's request. The latency in servicing the request depends on each
master's priority level and the responding peripheral's access time.
Since the crossbar switch appears to be just another slave to the master device, the master
device has no knowledge of whether it actually owns the slave port it is targeting. While
the master does not have control of the slave port it is targeting, it simply enters a wait
state.
A master is given control of the targeted slave port only after a previous access to a
different slave port completes, regardless of its priority on the newly targeted slave port.
This prevents deadlock from occurring when:
After the 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 or by leaving
that slave port for its next access. The master could also lose control of the slave port if
another higher priority master makes a request to the slave port; however, if the master is
running a fixed- or undefined-length burst transfer it retains control of the slave port until
that transfer completes.
The crossbar terminates all master IDLE transfers (as opposed to allowing the
termination to come from one of the slave busses). Additionally, when no master is
requesting access to a slave port the crossbar drives IDLE transfers onto the slave bus,
even though a default master may be granted access to the slave port.
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• A higher priority master has:
• A lower priority master is also making a request to the same slave port as the pending
access of the higher priority master.
• An outstanding request to one slave port that has a long response time and
• A pending access to a different slave port, and
MCF51JF128 Reference Manual, Rev. 2, 03/2011
Preliminary
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