Thread: Advanced (I think) pointer questions.

I am working with a STM32f1 micro. In the standard peripheral libraries, I noticed they use some very organized methods of accessing the many registers and peripherals. I do not understand a small portion of this.

Code:

typedef struct
{
  __IO uint32_t ACR;
  __IO uint32_t KEYR;
  __IO uint32_t OPTKEYR;
  __IO uint32_t SR;
  __IO uint32_t CR;
  __IO uint32_t AR;
  __IO uint32_t RESERVED;
  __IO uint32_t OBR;
  __IO uint32_t WRPR;
#ifdef STM32F10X_XL
  uint32_t RESERVED1[8]; 
  __IO uint32_t KEYR2;
  uint32_t RESERVED2;   
  __IO uint32_t SR2;
  __IO uint32_t CR2;
  __IO uint32_t AR2; 
#endif /* STM32F10X_XL */  
} FLASH_TypeDef;
#define FLASH               ((FLASH_TypeDef *) FLASH_R_BASE) 
/*******************  Bit definition for FLASH_CR register  ******************/
#define  FLASH_ACR_LATENCY                   ((uint8_t)0x03)               /*!< LATENCY[2:0] bits (Latency) */
#define  FLASH_CR_LATENCY_0                 ((uint8_t)0x00)               /*!< Bit 0 */
#define  FLASH_CR_LATENCY_1                 ((uint8_t)0x01)               /*!< Bit 0 */
#define  FLASH_CR_LATENCY_2                 ((uint8_t)0x02)               /*!< Bit 1 */
#define  FLASH_CR_HLFCYA                    ((uint8_t)0x08)               /*!< Flash Half Cycle Access Enable */
#define  FLASH_CR_PRFTBE                    ((uint8_t)0x10)               /*!< Prefetch Buffer Enable */
#define  FLASH_CR_PRFTBS                    ((uint8_t)0x20)               /*!< Control Register Status */

This code is used to access registers for micro operational control. I could write "FLASH -> CR &= FLASH_CR_LATENCY_0;" to set or clear bits. I understand how pointers are generally used to access memory bits, but I do not understand how this typedef is put before the asterisk in the FLASH define. Can anyone explain whats going on clearly? If I am not mistaken, the typedef structure is simply a means of using the correct size pointer to point the specific memory address, and nothing more. I really could replace the FLASH_TypeDef * with __IO uint32*, correct? By the way __IO is macro for volatile. Thanks much!

O_o

The relevant code is just a cast.

It says "Convert whatever type the variable `FLASH_R_BASE' really is into a `FLASH_TypeDef *' for my use.".

Soma

vart, I understand that this change would be made. But if you took that change one step farther, what code would replace (FLASH_TypeDef *)? Phantomotap, I am confused by your response. The variable FLASH_R_BASE is simply a #define.

Code:

#define FLASH_R_BASE          (AHBPERIPH_BASE + 0x2000) /*!< Flash registers base address */

It is simply a hex value address. Your response does not make sense to me.

Originally Posted by DanielB33

The variable FLASH_R_BASE is simply a #define.

So the code would cast the address (an integer) to be a pointer to the FLASH structure type:

Code:

((FLASH_TypeDef *) (AHBPERIPH_BASE + 0x2000)) -> CR &=  ((uint8_t)0x00);

It is simply a hex value address. Your response does not make sense to me.

O_o

I take it you haven't been working with embedded development for very long.

1): The macro `HBPERIPH_BASE' is the base address of the define range for targeting.
2): The `0x2000' is an offset from the base address to the necessary flash registers.
3): The value `AHBPERIPH_BASE + 0x2000' is just a hex value address which your compiler treats as an integer type value.
4): The statement `((FLASH_TypeDef *) (AHBPERIPH_BASE + 0x2000))' takes the integer type statement and casts the result into a pointer so that you can treat the calculated address as a pointer to a structure of type `FLASH_TypeDef'.

So, you are correct, the define is just an integer type value representing an address.

That is the problem.

Code:

(AHBPERIPH_BASE + 0x2000) -> CR &=  ((uint8_t)0x00);

This is wrong. An integer value is not a structure so contains no members.

The cast tells the compiler to treat the value as a pointer to a structure.

Code:

((FLASH_TypeDef *) (AHBPERIPH_BASE + 0x2000)) -> CR &=  ((uint8_t)0x00);

This is perfectly fine as `FLASH_TypeDef' is a structure with members.

Soma

FLASH_TypeDef is just an alias for the structure.

Code:

// you can name the struct as you would if you were not typedef'ing it
struct xyz {
   int a;
   int b;
};
// create the alias
typedef struct xyz xyz_typedef;

// declare some variables
struct xyz a;
xyz_typedef b;

// a and b refer to the same type. you can say
a = b; 
// ==================
// alternatively, you can combine the named structure and typedef
typedef struct xyz {
   int a;
   int b;
} xyz_typedef;


struct xyz a;
xyz_typedef b;

// a and b refer to the same type. you can say
a = b; 

// ==================
// now, if leave out the name of the structure, you just have the type 'xyz_typedef'

// note struct is not named now
typedef struct {
   int a;
   int b;
} xyz_typedef;


// struct xyz a; // this would give a compile error
xyz_typedef a;
xyz_typedef b;

// a and b refer to the same type. you can say
a = b;