19.3 Functional Description
19.3.1 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. Single-clock or zero-wait-state accesses are possible 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 slave's access time.
Because 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 waits.
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:
• A higher priority master has:
• An outstanding request to one slave port that has a long response time and
• A pending access to a different slave port, and
• A lower priority master is also making a request to the same slave port as the pending
access of the higher priority master.
After the master has control of the slave port it is targeting, the master remains in control
of the slave port until it relinquishes the slave port by running an IDLE cycle or by
targeting a different slave port for its next access.
The master can also lose control of the slave port if another higher-priority master makes
a request to the slave port.
The crossbar terminates all master IDLE transfers, as opposed to allowing the termination
to come from one of the slave buses. 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.
When a slave bus is being idled by the crossbar, it remains parked with the last master to
use the slave port. This is done to save the initial clock of arbitration delay that otherwise
would be seen if the master had to arbitrate to gain control of the slave port.
Functional Description
KL02 Sub-Family Reference Manual, Rev. 2.1, July 2013
258
Freescale Semiconductor, Inc.
