PM0223 datasheet(106/110 Pages) STMICROELECTRONICS

106 / 110 page

STM32L0 Core Peripherals

PM0223

106/110

DocID025763 Rev 1

STR R1, [R0, #0x0]

; Region Number

STR R4, [R0, #0x4]

; Region Base Address

STRH R2, [R0, #0x8]

; Region Size and Enable

STRH R3, [R0, #0xA]

; Region Attribute

Software must use memory barrier instructions:

•

Before MPU setup if there might be outstanding memory transfers, such as buffered

writes, that might be affected by the change in MPU settings.

•

After MPU setup if it includes memory transfers that must use the new MPU settings.

However, an instruction synchronization barrier instruction is not required if the MPU setup

process starts by entering an exception handler, or is followed by an exception return,

because the exception entry and exception return mechanism cause memory barrier

behavior.

For example, if you want all of the memory access behavior to take effect immediately after

the programming sequence, use a DSB instruction and an ISB instruction. A DSB is required

after changing MPU settings, such as at the end of context switch. An ISB is required if the

code that programs the MPU region or regions is entered using a branch or call. If the

programming sequence is entered using a return from exception, or by taking an exception,

then you do not require an ISB.

Subregions

Regions 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

on page 103. 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.

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 first 128KB region, set the

SRD field for region two to b00000011 to disable the first two subregions, as the figure

shows.

Figure 13. Example of SRD use

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