Table of Contents
Advertisement
13.5.1.2
Round-Robin Priority Operation
When operating in round-robin mode, each master is assigned a relative priority based on the master port
number. This priority is based on how far ahead the master port number of the requesting master is to the
master port number of the current bus master for this slave. Master port numbers are compared modulo the
total number of bus masters, i.e. take the requesting master port number minus the current bus master's
port number modulo the total number of bus masters. The master port with the highest priority based on
this comparison is granted control over the slave port at the next bus transfer boundary.
After granted access to a slave port, a master may perform as many transfers as desired to that port until
another master makes a request to the same slave port. The next master in line is granted access to the slave
port at the next transfer boundary.
Parking may continue to be used in a round-robin mode, but does 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.
13.5.1.3
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 the priority registers (XBS_PRSn) the crossbar
switch responds with a bus error (refer to
the registers are not updated.
13.6
Initialization/Application Information
No initialization is required by or for the crossbar switch. Hardware reset ensures all the register bits used
by the crossbar switch are properly initialized to a valid state. Settings and priorities should be
programmed to achieve maximum system performance.
Freescale Semiconductor
Section 12.2.5, "SCM Interrupt Status Register
MCF52277 Reference Manual, Rev. 1
Crossbar Switch (XBS)
(SCMISR)") and
13-7
Advertisement
Table of Contents
