How to declare a struct in CCM

[jean]

Posted on December 22, 2016 at 18:10

Hello,

I work on a STM32F4 and I want use the CCM memory to declare my structures.

For example I want this kind of structure to be always declared in the CCM memory:

typedef struct Assign_Pot Assign_Pot;

struct Assign_Pot {

uint8_t variable1;

uint8_t variable2;

uint16_t variable3;

};

Assign_Pot* Assign_Tab[100];

I've been looking across the forum but I can't find a solution that works.

If I declare my structure like this:

struct Assign_Pot {

uint8_t variable1;

uint8_t variable2;

uint16_t variable3;

} __attribute__ ((section('.ccm')));

I have the warning: 'section' attribute does not apply to types [-Wattributes]

And the structure is not created in the CCM

Do you have some experience to share with this problem?

Thanks a lot, have a great day!

Note: this post was migrated and contained many threaded conversations, some content may be missing.

[waclawek.jan]

Posted on December 23, 2016 at 01:25

The warning clearly tells you what's the problem: you can use the section attribute only when you declare variables, not when you create types. In other words, declaring a type (a struct tag here) only tells the compiler, what the variable looks like, not where variables of that type are going to be located, nor how many of them will be defined. OTOH if you define variable of whatever type, you tell the compiler to allocate a space in memory for that variable, so that's the right time to tell the compiler where exactly to allocate that space (normally you tell nothing and the compiler allocates it into its default space).

Nor does a pointer - either a pointer variable or a pointer type - define, where is the pointed-to object allocated, so defining pointers is not a way to tell the allocation of the pointed-to objects (you can allocate a pointer variable itself, though)

Declare a variable of struct Assign_Pot type and give it a section attribute.

JW

[Seb]

Posted on December 23, 2016 at 03:42

Normally the default linker configuration file (say IAR) would put the stack in the CCM if available.

As such, all local variables (struct based) declared inside a function should end up in the CCM, so developers benefits from it right away.

The main difference between main RAM and CCM is when lots of DMA transfer is going on.

DMA will take CPU cycles to read/write the main RAM. If using USB HS, SPI DMA, ADC/DAC DMA, then frequently accessed variables (or pointers) might be worth going to the CCM memory. Naturally, DMA accessed variables shouldn't be moved in the CCM. The lower the core frequency (on battery budget?) the more of a difference it could make.

Hope this help!

[jean]

Posted on December 23, 2016 at 11:31

Thanks a lot for all your tips!

, If I declare myvariable of

struct Assign_Pot

type and give it a section attribute, for example:

Assign_Pot* Assign_Tab[100] __attribute__ ((section('.ccm')));

It will only declare the 100

pointer in the CCM, not all the variables inside the structure, isn't it?

, how can be sure that my default linker configuration file put all my 'static' or 'global' variables end up in CCM memory? For your interest you can find attached the linker file I made. In my code, only the variables declared with

__attribute__ ((section('.ccm'))); are going into the CCM.

________________

Attachments :

cutom_link.ld.zip : https://st--c.eu10.content.force.com/sfc/dist/version/download/?oid=00Db0000000YtG6&ids=0680X000006Hz1Z&d=%2Fa%2F0X0000000bFM%2FtpDumIzm66M.y_H08VP2Os_FgbmFISqytecR4tOBk7I&asPdf=false

[Seb]

Posted on December 23, 2016 at 11:44

 ,

 ,

I'm an IAR toolchain user, in IAR there is a linker config file *.lnk which describes the various memory segments and name them.

To force a particular source code portion to put their code or variables in a specific segment, I use a ♯ pragma ,

Each compiler and linker have their own method to get the job done, this is where digging the doc becomes handy.

[jean]

Posted on December 23, 2016 at 12:31

Thank you. I use Coocox together with GNU Tools ARM Embedded, but it seems that I can't use 

♯ pragma (I found this doc 

http://www.st.com/content/ccc/resource/technical/document/application_note/bb/09/ca/83/14/e9/44/c5/DM00083249.pdf/files/DM00083249.pdf/jcr:content/translations/en.DM00083249.pdf

 )