Chapter 14 Crossbar Switch (XBAR)
MPC5602P Microcontroller Reference Manual, Rev. 4
278
Freescale Semiconductor
14.6.3
Master ports
A master access is taken if the slave port to which the access decodes is either currently servicing the
master or is parked on the master. In this case, the XBAR is completely transparent and the master access
is immediately transmitted on the slave bus and no arbitration delays are incurred. A master access stall if
the access decodes to a slave port that is busy serving another master, parked on another master.
If the slave port is currently parked on another master, and no other master is requesting access to the slave
port, then only one clock of arbitration is incurred. If the slave port is currently serving another master of
a lower priority and the master has a higher priority than all other requesting masters, then the master gains
control over the slave port as soon as the data phase of the current access is completed. If the slave port is
currently servicing another master of a higher priority, then the master gains control of the slave port after
the other master releases control of the slave port if no other higher priority master is also waiting for the
slave port.
A master access is responded to with an error if the access decodes to a location not occupied by a slave
port. This is the only time the XBAR directly responds with an error response. All other error responses
received by the master are the result of error responses on the slave ports being passed through the XBAR.
14.6.4
Slave ports
The goal of the XBAR with respect to the slave ports is to keep them 100% saturated when masters are
actively making requests. To do this the XBAR must not insert any bubbles onto the slave bus unless
absolutely necessary.
There is only one instance when the XBAR forces a bubble onto the slave bus when a master is actively
making a request. This occurs when a handoff of bus ownership occurs and there are no wait states from
the slave port. A requesting master that does not own the slave port is granted access after a one clock
delay.
14.6.5
Priority assignment
Each master port is assigned a fixed 3-bit priority level (hard-wired priority).
levels assigned to each master (the lowest has highest priority).
14.6.6
Arbitration
XBAR supports only a fixed-priority comparison algorithm.
Table 14-2. Hardwired bus master priorities
Module
Port
Priority level
Type
Number
e200z0 core–CPU instructions
Master
0
7
e200z0 core—Data
Master
1
6
eDMA
Master
2
5