PM0056 datasheet(110/156 Pages) STMICROELECTRONICS

110 / 156 page

Core peripherals

PM0056

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DocID15491 Rev 5

Subregions

Regions of 256 bytes or more are divided into eight equal-sized subregions. Set the

corresponding bit in the SRD field of the RASR to disable a subregion, see Section 4.2.9:

MPU region attribute and size register (MPU_RASR) on page 116. 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 the figure shows.

Figure 18. Subregion example

4.2.4

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 RASR, it must use aligned word accesses

•

For the 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.

Recommended MPU configuration

The STM32 microcontroller system has only a single processor, so you should program the

MPU as follows:

Region 1

Disabled subregion

Region 2, with

subregions

Base address of both regions

Offset from

base address

64KB

128KB

192KB

256KB

320KB

384KB

448KB

512KB

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