STM32F103C8 LED blink

Question

I try to program an STM32F103C8 circuit. I use an ST-LINK V2 programmer. After running a sample of code that used special libraries, I was able to see the built-in LED ON, but now I want to program the board without using those libraries, and I don't know why I cannot see anything, the LED is OFF the whole time. Here is the code:

#include "stm32f10x.h"                  // Device header

int main(){
    RCC->APB2ENR |= 1<<4;
    GPIOC->CRH &= ~0xFF0FFFF;
    GPIOC->CRH |= 0x00300000;
    GPIOC->ODR |= 1<<13;//turn PC_13 ON
    while(1){
        
    }
}

Answer 1

Based on the info that you have the LED on PC13, I assume you're using the famous Blue Pill board. In that board, LED connection is inverted because of the special limitations of PC13 - that pin should only sink current, not source. So, in order to turn it on, you need to write 0 to corresponding bit of ODR.

Normally, I'm against the usage of magic numbers . But for STM32F103, using hex literals for GPIO configuration is practical, because one single hex digit uniquely defines the pin mode. One can memorize the meaning of hex literals for GPIO settings, and use them in a compact assignment which defines the 8 pins of a GPIO port at once. If you make GPIO configurations only once, you don't need &= or |= operators.

Also, prefer using BSRR instead of ODR , because it allows atomic modification of output pins.

Here is a LED blink example for Blue Pill board:

//========================================================================
// Includes
//========================================================================
#include "stm32f1xx.h"
#include <stdbool.h>
#include <stdint.h>

//========================================================================
// Local Variables
//========================================================================
volatile uint32_t sysTick = 0;

//========================================================================
// Local Function Prototypes
//========================================================================
void initialize(void);
void delayMs(uint32_t ms);

//========================================================================
// Initialization
//========================================================================
void initialize(void) {

    SystemCoreClock = 8000000U;

    RCC->APB2ENR |= RCC_APB2ENR_IOPCEN // Enable PortC
            | RCC_APB2ENR_AFIOEN; // Enable A.F.Z

    // Pin Remaps
    AFIO->MAPR |= AFIO_MAPR_SWJ_CFG_JTAGDISABLE; // JTAG is disabled

    // GPIO Settings
    GPIOC->CRH = 0x44244444; // LED connected to PC13
    GPIOC->BSRR = (1 << 13); // LED is inverted, turn it off.

    // SysTick Settings
    SysTick_Config(SystemCoreClock / 1000);

    // Initialization Done Signal
    GPIOC->BSRR = (1 << (13 + 16));
    delayMs(200);
    GPIOC->BSRR = (1 << 13);
}

//========================================================================
// Main
//========================================================================
int main(void) {

    initialize();

    while (true) {
        delayMs(500);
        GPIOC->BSRR = (1 << (13 + 16));
        delayMs(500);
        GPIOC->BSRR = (1 << 13);
    }
}

//========================================================================
// Interrupt Handlers
//========================================================================

void SysTick_Handler(void)
{
    ++sysTick;
}

//========================================================================
// Local Functions
//========================================================================

void delayMs(uint32_t ms) {
    uint32_t start = sysTick;
    while (sysTick - start < ms);
}

Answer 2

#include "stm32f10x.h"                  // Device header

int main(void){
    RCC->APB2ENR |= 1<<4;
    GPIOC->CRH &= 0xFF0FFFF;//clear the necessary bit 
    GPIOC->CRH |= 0x00300000;//set the necessary bit
    GPIOC->ODR &= ~(1<<13);//turn PC_13 ON
        while(1){
        
    }
}

This code works. The problem was that PC13 controls the cathode, not the anode of the built-in LED. That's why I should write: GPIO->ODR &= ~(1 << 13); in order to turn ON the LED.