GPIO mode register

Question

I've adjusted the example from here for the STM3240G-EVAL board in order to blink LEDs 3 and 4. I have it working, but am confused by the Mode register setting:

GPIOG->MODER |= (GPIO_MODER_MODER6_0 | GPIO_MODER_MODER8_0) ;

When I read the reference manual (p186), it claims that the mode must be set to 01 for output, yet setting it to 0 in this way works just fine. Ideally I'd like to be able to change to the other modes, but I would have assumed that the above code would have changed pins 6 and 8 of port G to input pins. I must be missing something.

Here's my complete main document in case it's relevant:

#include "stm32f4xx.h"

/* We will use PG6 and PG8 connected to LEDs 1 and 2 because they're the same port. */
/* Find base register value for Port G                                              */


void delay (int a);

int main(void)
{
  /*!< At this stage the microcontroller clock setting is already configured,
       this is done through SystemInit() function which is called from startup
       file (startup_stm32f0xx.s) before to branch to application main.
       To reconfigure the default setting of SystemInit() function, refer to
       system_stm32f0xx.c file
    */

    /* GPIOG Periph clock enable */
RCC->AHB1ENR |= RCC_AHB1ENR_GPIOGEN;

GPIOG->MODER |= (GPIO_MODER_MODER6_0 | GPIO_MODER_MODER8_0) ;
    /* Configure PG6 and PG8 in output  mode  */

GPIOG->OTYPER &= ~(GPIO_OTYPER_OT_6 | GPIO_OTYPER_OT_8) ;
// Ensure push pull mode selected--default

GPIOG->OSPEEDR |= (GPIO_OSPEEDER_OSPEEDR6|GPIO_OSPEEDER_OSPEEDR8);
//Ensure maximum speed setting (even though it is unnecessary)

GPIOG->PUPDR &= ~(GPIO_PUPDR_PUPDR6|GPIO_PUPDR_PUPDR8);
//Ensure all pull up pull down resistors are disabled

while (1)
{
    /* Set PG6 and PG8 */
    /* the bit set/reset low register SETS the output data register     */
    /* the bit set/reset high register RESETS the output data register  */

    GPIOG -> BSRRL = (1 << 6);
    GPIOG -> BSRRL = (1 << 8);
    delay(500000);
    /* Reset PC8 and PC9 */
    GPIOG -> BSRRH = (1 << 6);
    GPIOG -> BSRRH = (1 << 8);
    delay(500000);
    }

return 0;
} 

void delay (int a)
{
volatile int i,j;

for (i=0 ; i < a ; i++)
{
    j++;
}

return;
}

Answer 1

You aren't setting it to zero, you're setting it to one.

The definition of the GPIO_MODER_MODER6_0 constant is 0x00001000. The mask for the GPIO_MODER_MODER6 bits is 0x00003000, so you're putting bits 01 into the right place.

If the constant GPIO_MODER_MODER6_0 were defined as zero, then or'ing it into the configuration register would have no effect in any case. To set both bits to zero you'd need to do something like:

GPIOG->MODER &= ~(GPIO_MODER_MODER6_0 | GPIO_MODER_MODER6_1);

The _0 and _1 suffixes refer to the bit numbers for masking, not the values being written.