Crossbar Switch (XBS)
MCF52277 Reference Manual, Rev. 1
Freescale Semiconductor
13-7
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_PRS
n
) the crossbar
switch responds with a bus error (refer to
Section 12.2.5, “SCM Interrupt Status Register (SCMISR)”
) and
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.
Summary of Contents for MCF52277
Page 22: ...MCF52277 Reference Manual Rev 1 Freescale Semiconductor xxii...
Page 32: ...MCF52277 Reference Manual Rev 0 xxxii Freescale Semiconductor...
Page 60: ...Signal Descriptions MCF52277 Reference Manual Rev 1 2 16 Freescale Semiconductor...
Page 128: ...Static RAM SRAM MCF52277 Reference Manual Rev 1 6 6 Freescale Semiconductor...
Page 140: ...Clock Module MCF52277 Reference Manual Rev 1 7 12 Freescale Semiconductor...
Page 172: ...Chip Configuration Module CCM MCF52277 Reference Manual Rev 1 9 16 Freescale Semiconductor...
Page 180: ...Serial Boot Facility SBF MCF52277 Reference Manual Rev 1 10 8 Freescale Semiconductor...
Page 188: ...Reset Controller Module MCF52277 Reference Manual Rev 1 11 8 Freescale Semiconductor...
Page 210: ...Crossbar Switch XBS MCF52277 Reference Manual Rev 1 13 8 Freescale Semiconductor...
Page 542: ...FlexCAN MCF52277 Reference Manual Rev 1 23 32 Freescale Semiconductor...
Page 724: ...UART Modules MCF52277 Reference Manual Rev 1 30 34 Freescale Semiconductor...
Page 740: ...I2 C Interface MCF52277 Reference Manual Rev 1 31 16 Freescale Semiconductor...
Page 790: ...Debug Module MCF52277 Reference Manual Rev 1 32 50 Freescale Semiconductor...