[En-Nut-Discussion] STM32F7 RAM

[Holger Mai]

Ok, lets have a look into the STM32F7xx reference manual, chapter 2.3 Embedded
SRAM.

The ITCM is for instructions only, and only accessible from core. It can be used
for (very fast) code, but before the code must written into by the core. ITCM is
the best choice to run Code from RAM (because no waitstates, but this is only
possible if the code is time critical, i.e. much iterations/loops for
calculations). Therefore it can be used as a special section in the
linkerscript.

This meets your points "RAM functions" and "Code when running from RAM
(NUTDEBUG_RAM, text section)"

Stacks are tricky because you must guarant, that no local variables are written
or readed with any DMAs. I think this should be avoided. On the other hand, 16kb
are not so much.

Next are the two Blocks SRAM1 and SRAM2. This is the same RAM type, and a
separation makes only sense, if you had to to such things like double buffering
for time critical data in/output. For those setups you must refer to the Bus
Matrix in the ref-Manual.

However, this is the Memory for an "universal" usage.

Last is DTCM: "Data Tightly Coupled Memory" is connected directly to the core,
and accessible over the Busmatrix, but only over a special interface in the
core, and over the same bus that is used for access from core. I found no
details how this is touched by some things like data caching and access
prioritys. There are some tests necessary to evaluate these circumstances.
Therefore i doesnt use DTCM at the moment for heap, only as section for static
variables. If there a dynamic memory management for this section aside the
normal malloc were possible, then it can be easy used as memory for special
usage.

I think that DTCM is coupled with the core about a 64bit interface (because a
bold line in the Memory architecture sheet, see RefMan 2.1), but the RefManual
doesnt say this explicit (for the ITCM Bus it does).

An additional Feature of the DTCM is a clock source, who can enable/disable the
DTCM Clock (AHB1, DTCMRAMEN). This maybe equal with an access lock, but the
Refmanual says nothing about data preservation if disabled and then reenabled.
This clock must be enabled by default on startup time.



mit freundlichen Grüßen /Best Regards

Holger Mai

mai at gemac-chemnitz.de



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