Fixed-Priority Operation; Round-Robin Priority Operation - NXP Semiconductors freescale KV4 Series Reference Manual

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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

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

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.

The requesting master's priority level is lower than the current

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.

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At the next clock edge

At the conclusion of one of the following cycles:

• An IDLE cycle

• A non-IDLE cycle to a location other than the current

KV4x Reference Manual, Rev. 2, 02/2015

Chapter 20 Crossbar Switch Lite (AXBS-Lite)

Then the Crossbar Switch grants control to the

requesting master