Cortex-M3 Processor (Reference Material)
UG0331 User Guide Revision 15.0
130
; R1 = address and region number in one
; R2 = size and attributes in one
LDR R0, =MPU_MPU_RBAR
; 0xE000ED9C, MPU Region Base register
STR R1, [R0, #0x0]
; Region base address and
; region number combined with VALID (bit 4) set to 1
STR R2, [R0, #0x4]
; Region Attribute, Size and Enable
3.7.4.8.1
Subregions
Regions of 256 bytes or more are divided into eight equal-sized subregions. Set the corresponding bit in
the SRD field of the MPU_RASR to disable a subregion, see
MPU Region Attribute and Size Register,
page 127. The least significant bit of SRD controls the first subregion, and the most significant bit
controls the last subregion. Disabling a subregion means another region overlapping the disabled range
matches instead. If no other enabled region overlaps the disabled subregion the MPU issues a fault.
Regions of 32, 64, and 128 bytes do not support subregions, With regions of these sizes, you must set
the SRD field to 0x00, otherwise the MPU behavior is Unpredictable.
Example of SRD use
Two regions with the same base address overlap. Region one is 128KB, and region two is 512KB. To
ensure the attributes from region one apply to the first128KB region, set the SRD field for region two to
b00000011 to disable the first two subregions, as shown in the following figure.
Figure 55 •
SRD Field
3.7.4.9
MPU Design Hints and Tips
To avoid unexpected behavior, disable the interrupts before updating the attributes of a region that the
interrupt handlers might access.
Ensure software uses aligned accesses of the correct size to access MPU registers:
•
except for the MPU_RASR, it must use aligned word accesses
•
for the MPU_RASR it can use byte or aligned halfword or word accesses.
The processor does not support unaligned accesses to MPU registers.
When setting up the MPU, and if the MPU has previously been programmed, disable unused regions to
prevent any previous region settings from affecting the new MPU setup.
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