MPC563XM Reference Manual, Rev. 1
194
Freescale Semiconductor
Preliminary—Subject to Change Without Notice
Arbitration always occurs on a clock edge, but only occurs on edges when a change in mastership will not
violate AHB-Lite protocals. Valid arbitrations points include any clock cycle in which
sX_hready
is
asserted (provide the master is not performing a burst or locked cycle) and any wait state in which the
master owning the bus indicates a transfer type of IDLE (provided the master is not performing a locked
cycle).
Since arbitration can occur on every clock cycle the slave port masks off all master requests if the current
master is performing a locked transfer or a protected burst transfer, guaranteeing that no matter how low
its priority level it will be allowed to finish its locked or protected portion of a burst sequence.
8.4.4.4.3
Slave Port State Machine Master Handoff
The only times the slave port will switch masters when programmed for fixed priority mode of operation
is when a higher priority master makes a request or when the current master is the highest priority and it
gives up the slave port by either running and IDLE cycle to the slave port or running a valid access to a
location other than the slave port.
If the current master loses control of the slave port because a higher priority master takes it away the slave
port will not incur any wasted cycles. The current master will get its current cycle terminated by the slave
port at the same time the new master’s address and control information will be recognized by the slave
port. This will look like a seamless transition on the slave port.
If the current master is being wait stated when the higher priority master makes its request, then the current
master will be allowed to make one more transaction on the slave bus before giving it up to the new master.
illustrates the effect of a higher priority master taking control of the bus when the slave port
is programmed for a fixed priority mode of operation.
Figure 8-10. Low to high priority mastership change
1
2
3
4
5
6
7
8
9
Master 5 Master 5 Master 4 Master 3 Master 2
Master 3
Master 4
None
XBAR
Master 5
Master 5
Master 2
Master 3 Master 4
XBAR
IDLE
NSEQ
NSEQ
NSEQ
NSEQ
NSEQ
IDLE
hclk
m2 request
m3 request
m4 request
m5 request
htrans
hready
Requester
Priority
Highest
Address/Cntrl
owner
10