embeddedgpioledcmsisstm32f1

Setting GPIOC->ODR[13] = 1 is not tuning up the user LED, rather it is turned up by leaving it to default (i.e. 0). [STM32F103C8T6 (blue pill)]

I have recently started learning bare metal embedded development using CMSIS Core framework. I do not understand why the user LED is not turning on by setting the ODR[13] to 1 but rather it is turned on by leaving it to default i.e. 0.

Here is the code I have written.

 #include "stm32f103xb.h"


int main(void){
    /** 
    * Enable clock for APB2 bus
    * RCC_AP2ENR --> BIT 4 --> 1
    **/
    RCC->APB2ENR |= RCC_APB2ENR_IOPCEN;

    /**
     * Set GPIOC PIN 13 OUTPUT
     * GPIOC_CRH --> BIT(23,22,21,20) = (0,0,0,1)
     * MODE13[1:0] = 0 1
     * CNF13[1:0] = 0 0
     */
    GPIOC->CRH = GPIO_CRH_MODE13_0 | (GPIOC->CRH & ~ GPIO_CRH_CNF13);
    
    while(1){
        GPIOC->ODR |= GPIO_ODR_ODR13; // commenting this line turns on the LED
    }
}
Solution

  • Disclaimer: I have a software background, and I am using the STM32F103C8T6 blue pill as a hobbyist.

    This being said, according to stm32-base.org, it seems that the user LED on PC13 is wired in sink mode. This would explain why setting ODR[13]to 0 is powering the LED on: this does allow the current to flow through the load - see the schematic on the stm32-base.org page:

    enter image description here