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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.
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 same master had to arbitrate to gain control of the slave port.
17.4.2 Arbitration
The crossbar switch supports two arbitration algorithms:
• Fixed priority
• Round-robin
The selection of the global slave port arbitration is controlled in the MCM module. For
fixed priority, set MCM_PLACR[ARB] to 0. For round robin, set MCM_PLACR[ARB]
to 1. This arbitration setting applies to all slave ports.
17.4.2.1 Arbitration during undefined length bursts
All lengths of burst accesses lock out arbitration until the last beat of the burst.
Freescale Semiconductor, Inc.
KL27 Sub-Family Reference Manual , Rev. 5, 01/2016
Chapter 17 Crossbar Switch Lite (AXBS-Lite)
259
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