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Chapter 21 Memory Protection Unit (MPU)
MPC5606BK Microcontroller Reference Manual, Rev. 2
392
Freescale Semiconductor
•
Performing the required logic functions to force the standard 2-cycle AHB error response to
properly terminate the bus transaction and then providing the right values to the crossbar switch to
commit the transaction to other portions of the platform.
If, instead, the access is allowed, then the MPU simply passes all original signals to the slave device. In
this case, from a functionality point of view, the MPU is fully transparent.
21.7
Initialization information
The reset state of MPU_CESR[VLD] disables the entire module. Recall that while the MPU is disabled,
all accesses from all bus masters are allowed. This state also minimizes the power dissipation of the MPU.
The power dissipation of each access evaluation macro is minimized when the associated region descriptor
is marked as invalid or when MPU_CESR[VLD] = 0.
Typically the appropriate number of region descriptors (MPU_RGD
n
) is loaded at system startup,
including the setting of the MPU_RGDn.Word3[VLD] bits, before MPU_CESR[VLD] is set, enabling the
module. This approach allows all the loaded region descriptors to be enabled simultaneously. Recall if a
memory reference does not hit in any region descriptor, the attempted access is terminated with an error.
21.8
Application information
In an operational system, interfacing with the MPU can generally be classified into the following activities:
•
Creation of a new memory region requires loading the appropriate region descriptor into an
available register location. When a new descriptor is loaded into a RGD
n
, it would typically be
performed using four 32-bit word writes. As discussed in
Section 21.5.2.4.4, MPU Region
Descriptor n, Word 3 (MPU_RGDn.Word3)
, the hardware assists in the maintenance of the valid
bit, so if this approach is followed, there are no coherency issues associated with the multi-cycle
descriptor writes. Deletion/removal of an existing memory region is performed simply by clearing
MPU_RGD
n
.Word3[VLD].
•
If only the access rights for an existing region descriptor need to change, a 32-bit write to the
alternate version of the access control word (MPU_RGDAAC
n
) would typically be performed.
Recall writes to the region descriptor using this alternate access control location do not affect the
valid bit, so there are, by definition, no coherency issues involved with the update. The access
rights associated with the memory region switch instantaneously to the new value as the IPS write
completes.
•
If the region’s start and end addresses are to be changed, this would typically be performed by
writing a minimum of three words of the region descriptor: MPU_RGD
n
.Word{0,1,3}, where the
writes to Word0 and Word1 redefine the start and end addresses, respectively, and the write to
Word3 re-enables the region descriptor valid bit. In many situations, all four words of the region
descriptor would be rewritten.
•
Typically, references to the MPU’s programming model would be restricted to supervisor mode
accesses from a specific processor(s), so a region descriptor would be specifically allocated for this
purpose with attempted accesses from other masters or while in user mode terminated with an error.
When the MPU detects an access error, the current bus cycle is terminated with an error response, and
information on the faulting reference is captured in the MPU_EARn and MPU_EDR
n
registers. The
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