GPIO struct parameters not initialized

[chrisevans3]

Posted on August 05, 2011 at 12:26

The GPIO struct is defined but it is not initialized before it is used in main.  Everything works but I am stuck with trying to find a value that I can use in GPIO_Init.  GPIOx->CR2 is OR'd with another value and I don't know where to get the initial value GPIOx->CR2.  The only thing I can think is that a random number is in that address.

This is in the given program that comes with the firmware.  I am new to C and this is probably a simple problem. 

Thanks,

Chris

In the main function that comes with the Discovery microcontroller.  The first thing that main does is to call GPIO_Init.   I have included it here.

void GPIO_Init(GPIO_TypeDef* GPIOx,

               uint8_t GPIO_Pin,

               GPIO_Mode_TypeDef GPIO_Mode)

{

 

 

  /*----------------------*/

  /* Check the parameters */

  /*----------------------*/

  assert_param(IS_GPIO_MODE(GPIO_Mode));

  assert_param(IS_GPIO_PIN(GPIO_Pin));

  /* Reset crresponding bit to GPIO_Pin in CR2 register */

  GPIOx->CR2 &= (uint8_t)(~(GPIO_Pin));

  /*-----------------------------*/

  /* Input/Output mode selection */

  /*-----------------------------*/

  if ((((uint8_t)(GPIO_Mode)) & (uint8_t)0x80) != (uint8_t)0x00) /* Output mode */

  {

    if ((((uint8_t)(GPIO_Mode)) & (uint8_t)0x10) != (uint8_t)0x00) /* High level */

    {

      GPIOx->ODR |= GPIO_Pin;

    } else /* Low level */

    {

      GPIOx->ODR &= (uint8_t)(~(GPIO_Pin));

    }

    /* Set Output mode */

    GPIOx->DDR |= GPIO_Pin;

  } else /* Input mode */

  {

    /* Set Input mode */

    GPIOx->DDR &= (uint8_t)(~(GPIO_Pin));

  }

I think you can see that GPIOx->CR2 and GPIOx->ODR and GPIOx->DDR are being used to set bits.(Outlined in RED)  My thinking is that they are not initialized by the function GPIO_Init yet so any number that happens to be in that address could be used.

I am a beginner in both uC's and C.  Thanks for your help.

Chris

#initialize-a-structure #memory-mapped-peripherals

[Andrew Neil]

Posted on August 06, 2011 at 09:00

Can you show an example to illustrate what you're talking about?

''I am new to C''

Are you also new to programming in general, and/or to microcontrollers, and/or digital electronics?

[chrisevans3]

Posted on August 08, 2011 at 18:27

[Andrew Neil]

Posted on August 09, 2011 at 08:09

''Thanks for responding to my post.''

You're welcome. 

''I am new to C and uC's''

 

Don't try to learn too much new stuff all at once!

It might be easier to learn the 'C' programming language on a PC or other ''normal'' platform before moving on to apply it to microcontrollers...

Some 'C' starting tips here: 

http://blog.antronics.co.uk/2011/08/08

''have some experience in digital electronics''

So are you familiar with the concept of memory-mapped IO?

It means that the IO registers appear in the CPU's normal memory map as normal memory locations - so the CPU accesses the peripherals just with normal memory reads & writes.

Therefore, by locating 'C' variables at these locations, your 'C' program can access the peripheral registers by just accessing the variables.

This is what the code you showed is doing - using a struct to group all the registers of a port together.

Therefore you do not initialise the structure - reading the structure just gives the values from the hardware registers!

[Andrew Neil]

Posted on August 09, 2011 at 10:36

''memory-mapped IO ... using a struct to group all the registers of a port together''

Note that this is a very common & widely used technique.

[chrisevans3]

Posted on August 09, 2011 at 12:15