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MPC5553/MPC5554 Microcontroller Reference Manual, Rev. 5
5-12
Freescale Semiconductor
5.4
Functional Description
The PBRIDGE serves as an interface between a system bus and the peripheral (slave) bus. It functions as
a protocol translator. Support is provided for generating a pair of 32-bit peripheral accesses when targeted
by a 64-bit system bus access. No other bus-sizing access support is provided.
Accesses that fall within the address space of the PBRIDGE are decoded to provide individual module
selects for peripheral devices on the slave bus interface.
5.4.1
Access Support
Aligned 64-bit accesses, aligned word and halfword accesses, as well as byte accesses are supported for
32-bit peripherals. Peripheral registers must not be misaligned, although no explicit checking is performed
by the PBRIDGE.
NOTE
Data accesses that cross a 32-bit boundary are not supported.
5.4.2
Peripheral Write Buffering
The PBRIDGE provides programmable write buffering capability to allow certain write accesses to be
buffered in the PBRIDGE for later completion, while terminating the system bus access early. This
provides improved performance in systems where frequent writes to a slow peripheral are performed.
Write buffering must only be enabled for masters and peripherals for which an error termination from the
slave bus will either not occur, or is safe to ignore.
When write buffering is enabled, all accesses through the PBRIDGE will still occur in-order; no bypassing
of buffered writes is supported.
Write buffering is controllable on a per-master and per-peripheral basis.
NOTE
Write buffering will cause the processor core to believe that the write has
completed before it actually has completed in the peripheral. If write
buffering is enabled for a peripheral, the actual write will take an additional
two system clock cycles plus any additional system clock cycles that the
register needs. Most registers in the MPC5500 family only will delay the
write by two clock cycles, but some registers will take longer. This early
termination, as seen by the processor core, can defeat the
mbar
or
msync
instruction between the write to clear a flag bit and the write to the
INTC_EOIR. Refer to
Section 10.4.3.1.2, “End-of-Interrupt Exception
.” Therefore, if write buffering is enabled for a peripheral that has a
flag bit, insert instructions between the
mbar
or
msync
instruction and the
write to the INTC_EOIR that will consume at least the number of system
clock cycles that the actual write is delayed.
5.4.2.1
Read Cycles
Read accesses are possible with the PBRIDGE when the requested access size is 32-bits or smaller, and is
not misaligned across a 32-bit boundary. 64-bit data reads (not instruction) are not supported.
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