Crossbar Switch (XBAR)
MPC5566 Microcontroller Reference Manual, Rev. 2
Freescale Semiconductor 7-11
As an example of arbitration in round-robin mode, assume the three masters have ID’s 0, 1, and 2. If the
last master of the slave port was master 1, and masters 0 and 2 make simultaneous requests, they are
serviced in the order 2 and then 0 assuming no further requests are made.
As another example, if master 1 is waiting on a response from a slow slave and has no further pending
access to that slave, no other masters are requesting, and master 0 then makes a request, master 0’s request
is granted on the next clock (assuming that master 1’s transfer is not a burst transfer), and the request
information for master 0 is driven to the slave as a pending access. If master 2 were to make a request after
master 0 has been granted access, but prior to master 0’s access being accepted by the slave, master 0
maintains the grant on the slave port, and master 2 is delayed until the next arbitration boundary, which
occurs after the transfer is complete. The round-robin pointer is reset to 0, so if master 1 has another request
that occurs before master 0’s transfer completes, master 1 is the granted the bus. This implies a worst case
latency of N transfers for a system with N masters.
Parking may continue to be used in round-robin mode, but affects the round-robin pointer unless the
parked master actually performs a transfer. Handoff to the next master in line occurs after one cycle of
arbitration.
The slave port does an arbitration check at every clock edge to ensure that the proper master (if any) has
control of the slave port.
A new requesting master must wait until the end of the fixed-length burst transfer, before it is granted
control of the slave port. If the new requesting master’s priority level is lower than that of the master that
currently has control of the slave port, the new requesting master is forced to wait until the master that
currently has control of the slave port completes its access.
7.3.6.2.1 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.
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