NXP Semiconductors MPC5644A, Reference Manual, Page: 203 / 1735

Multi-Layer AHB Crossbar Switch (XBAR)
MPC5644A Microcontroller Reference Manual, Rev. 6
Freescale Semiconductor 203
As an example of arbitration in round-robin mode, assume the XBAR is implemented with master ports 0,
1, 4 and 5. If the last master of the slave port was master 1, and master 0, 4 and 5 make simultaneous
requests, they will be serviced in the order 4, 5 and then 0.
Parking may still be used in a round-robin mode, but will not affect the round-robin pointer unless the
parked master actually performs a transfer. Handoff occurs to the next master in line after one cycle of
arbitration. If the slave port is put into low power park mode the round-robin pointer is reset to point at
master port 0, giving it the highest priority.
9.3.1.3 Parking
If no master is currently requesting the slave port, the slave port is parked. The slave port parks in one of
three places, indicated by the value of the PCTL field in the XBAR_SGPCR.
• If park-on-specific master mode is selected, the slave port parks on the master designated by the
PARK field. When the master accesses the slave port again, a one clock arbitration penalty is
incurred only for an access request made by another master port to the slave port. No other
arbitration penalties are incurred. All other masters pay a one clock penalty.
• If park-on-last (POL) mode is selected, then the slave port parks on the last master to access it,
passing that master’s signals through to the slave bus. When the master accesses the slave port
again, no other arbitration penalties are incurred except that a one clock arbitration penalty is
incurred for each access request to the slave port made by another master port. All other masters
pay a one clock penalty.
• If the low-power-park (LPP) mode is selected, then the slave port enters low-power park mode. It
is not under control by any master and does not transmit any master signals to the slave bus. All
slave bus activity halts because all slave bus signals are not toggling. This saves power if the slave
port is not used for some time. However, when a master does make a request to a slave port parked
in low-power-park, a one clock arbitration delay is incurred to get ownership of the slave port.
9.3.2 Priority assignment
Each master port needs to be assigned a unique 3-bit priority level. If an attempt is made to program
multiple master ports with the same priority level within a register (MPR) the XBAR will respond with an
error and the registers will not be updated.