20.4.2.2 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 20-1. 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
The requesting master's priority level is lower than the current
master.
At the conclusion of one of the following cycles:
• An IDLE cycle
• A non-IDLE cycle to a location other than the current
slave port
20.4.2.3 Round-robin priority operation
When operating in round-robin mode, each master is assigned a relative priority based on
the master port number. This relative priority is compared to the master port number (ID)
of the last master to perform a transfer on the slave bus. The highest priority requesting
master becomes owner of the slave bus at the next transfer boundary. Priority is based on
how far ahead the ID of the requesting master is to the ID of the last master.
Chapter 20 Crossbar Switch Lite (AXBS-Lite)
KV4x Reference Manual, Rev. 2, 02/2015
Freescale Semiconductor, Inc.
Preliminary
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