Information

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.
18.3.2 Arbitration
The crossbar switch supports two arbitration schemes:
• A fixed-priority comparison algorithm
• A round-robin fairness algorithm
18.3.2.1 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 (master 1 has lower priority
than master 3). If two masters request access to a slave port, the master with the highest
priority gains control over the slave 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 18-1. How AXBS grants control of a slave port to a master
When Then AXBS 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:
• The current master is running a fixed length burst
transfer or a locked transfer.
• The requesting master's priority level is higher than that
of the current master.
At the end of the burst transfer or locked transfer
Table continues on the next page...
Chapter 18 Crossbar Switch Lite (AXBS-Lite)
K20 Sub-Family Reference Manual, Rev. 2, Feb 2012
Freescale Semiconductor, Inc. 315