AN4838 S field equivalent to non-cacheable

[GreenGuy]

Posted on May 17, 2018 at 00:50

In AN4838 it states on page 7/16:

The STM32F7 Series and

STM32H7 Series do not support hardware coherency. the S field is equivalent to non-cacheable memory.

Aside from the typo, I find this information to be confusing.  Does this mean that in F/H7 series devices that the S bit determines the cache behavior and is inter changeable with the C bit?  Or is it ignored?  Or is the S field and the S bit two different things?

Please clarify?

[Khouloud GARSI]

Posted on May 17, 2018 at 13:07

Hello

greenwood.greg

‌,

The Shared attribute S is handled differently between Cortex-A and Cortex-M: In cortex-A, enabling S bit will enable Hardware coherency. However, cortex-Mdoesn’t support Hardware coherency snooping. So,enabling the S bit will automatically disable the cache.

Khouloud.

[GreenGuy]

Posted on May 17, 2018 at 18:20

So then using a statement like:

MPU_InitStruct.IsShareable = MPU_ACCESS_SHAREABLE;

will override:

MPU_InitStruct.IsCacheable = MPU_ACCESS_CACHEABLE;

in the same region making that region NOT cacheable.

In the same vein:

MPU_InitStruct.IsCacheable = MPU_ACCESS_NOT_CACHEABLE;

MPU_InitStruct.IsShareable = MPU_ACCESS_SHAREABLE;

in the same region is redundant.

Would that be correct?

[Khouloud GARSI]

Posted on May 18, 2018 at 12:02

Yes, correct.