MPC5553/MPC5554 Microcontroller Reference Manual, Rev. 5
Freescale Semiconductor
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to the slave port when the current transfer is completed, or possibly on the next clock cycle if the current
master has no pending access request.
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 will be
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
will be granted on the next clock (assuming that master 1’s transfer is not a burst transfer), and the request
information for master 0 will be 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 will continue to be granted the slave port, and master 2 will be delayed until the next arbitration
boundary, which occurs after the transfer is complete. The round-robin pointer will have been reset to 0,
so master 1 could actually be granted the bus next if it has another request which occurs prior to the
completion of master 0’s transfer. This implies a worst case latency of N transfers for a system with N
masters.
Parking may still be used in round-robin mode, but will not affect the round-robin pointer unless the parked
master actually performs a transfer. Handoff will occur to the next master in line 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 will be granted
control of the slave port. But 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 will be forced to wait until the master
that currently has control of the slave port is finished accessing the current slave port.
7.3.6.2.1
Parking
If no master is currently making a request to the slave port then the slave port will be parked. It will park
in one of three places, dictated by the PCTL field in the XBAR_SGPCR.
•
If the park on specific master mode is selected, then the slave port will park on the master
designated by the PARK field. The behavior here is the same as for the POL mode with the
exception that a specific master will be parked on instead of the last master to access the slave port.
If the master designated by the PARK field tries to access the slave port it will not pay an arbitration
penalty, while any other master will pay a one clock penalty.
•
If the park on last (POL) mode is selected, then the slave port will park on the last master to access
it, passing that master’s signals through to the slave bus. When that master accesses the slave port
again it will not pay any arbitration penalty; however, if any other master wishes to access the slave
port a one clock arbitration penalty will be imposed.
•
If the low power park (LPP) mode is selected, then the slave port will enter low power park mode.
It will not recognize any master as being in control of it and it will not select any master’s signals
to pass through to the slave bus. In this case all slave bus activity will effectively halt because all
slave bus signals will not be toggling. This can save power if the slave port will not be in use for
Summary of Contents for MPC5553
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