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continued burst, or the ninth beat of the second burst from the master's perspective, is
taken, all beats of the burst are once again open for arbitration until the master loses
control of the slave port.
Assume the master again loses control of the slave port on the fifth beat of the third now
continued burst, or the 10th beat of the second burst from the master's perspective. After
the master regains control of the slave port, it is allowed to complete its final two beats of
its burst without facing arbitration.
Note
Fixed-length burst accesses are not affected by the AULB bits.
All fixed-length burst accesses lock out arbitration until the last
beat of the fixed-length burst.
18.3.3.2 Fixed-priority operation
When operating in Fixed-Priority mode, each master is assigned a unique priority level in
the priority registers (PRSn) . If two masters request access to a slave port, the master
with the highest priority in the selected priority register 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 18-29. 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
Both of the following are true:
• The current master is running an undefined length burst
transfer.
• The requesting master's priority level is higher than that
of the current master.
At the next arbitration point for the undefined length burst
transfer
NOTE: Arbitration points for an undefined length burst are
defined in the MGPCR for each master.
Table continues on the next page...
Chapter 18 Crossbar Switch (AXBS)
K20 Sub-Family Reference Manual, Rev. 1.1, Dec 2012
Freescale Semiconductor, Inc.
Preliminary
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