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19.3.2 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.
19.3.2.1 Fixed-priority operation
When operating in fixed-priority mode, each master is assigned a unique priority level
with the highest numbered master having the highest priority (for example, in a system
with 5 masters, master 1 has lower priority than master 3). If two masters request access
to the same slave port, the master with the highest priority gains control over the slave
port.
NOTE
In this arbitration mode, a higher-priority master can
monopolize a slave port, preventing accesses from any lower-
priority master to the port.
When a master makes a request to a slave port, the slave port checks whether the new
requesting master's priority level is higher than that of the master that currently has
control over the slave port, unless the slave port is in a parked state. The slave port
performs an arbitration check at every clock edge to ensure that the proper master, if any,
has control of the slave port.
The following table describes possible scenarios based on the requesting master port:
Table 19-1. How the Crossbar Switch grants control of a slave port to a master
When
Then the Crossbar Switch grants control to the
requesting master
Both of the following are true:
• The current master is not running a transfer.
• The new requesting master's priority level is higher than
that of the current master.
At the next clock edge
Both of the following are true:
At the next arbitration point for the undefined length burst
transfer
Table continues on the next page...
Chapter 19 Crossbar Switch Lite (AXBS-Lite)
K22F Sub-Family Reference Manual, Rev. 4, 08/2016
NXP Semiconductors
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