blink.c

// Blink the LED connected to Pin PC13 on an STM32 Blue Pill
// From https://satoshinm.github.io/blog/171212_stm32_blue_pill_arm_development_board_first_look_bare_metal_programming.html

#define STACK_TOP 0x20002000
#include "stm32f10x.h"

// from: stm32f103xb.h
#define RCC_APB2ENR_IOPAEN_Pos               (2U)                              
#define RCC_APB2ENR_IOPAEN_Msk               (0x1U << RCC_APB2ENR_IOPAEN_Pos)  /*!< 0x00000004 */
#define RCC_APB2ENR_IOPAEN                   RCC_APB2ENR_IOPAEN_Msk            /*!< I/O port A clock enable */
#define RCC_APB2ENR_IOPCEN_Pos               (4U)                              
#define RCC_APB2ENR_IOPCEN_Msk               (0x1U << RCC_APB2ENR_IOPCEN_Pos)  /*!< 0x00000010 */
#define RCC_APB2ENR_IOPCEN                   RCC_APB2ENR_IOPCEN_Msk            /*!< I/O port C clock enable */

#define GPIO_CRH_MODE13_Pos                  (20U)                             
#define GPIO_CRH_MODE13_Msk                  (0x3U << GPIO_CRH_MODE13_Pos)     /*!< 0x00300000 */
#define GPIO_CRH_MODE13                      GPIO_CRH_MODE13_Msk               /*!< MODE13[1:0] bits (Port x mode bits, pin 13) */
#define GPIO_CRH_MODE13_0                    (0x1U << GPIO_CRH_MODE13_Pos)     /*!< 0x00100000 */
#define GPIO_CRH_MODE13_1                    (0x2U << GPIO_CRH_MODE13_Pos)     /*!< 0x00200000 */

#define GPIO_CRH_CNF13_Pos                   (22U)
#define GPIO_CRH_CNF13_Msk                   (0x3U << GPIO_CRH_CNF13_Pos)      /*!< 0x00C00000 */
#define GPIO_CRH_CNF13                       GPIO_CRH_CNF13_Msk                /*!< CNF13[1:0] bits (Port x configuration bits, pin 13) */
#define GPIO_CRH_CNF13_0                     (0x1U << GPIO_CRH_CNF13_Pos)      /*!< 0x00400000 */
#define GPIO_CRH_CNF13_1                     (0x2U << GPIO_CRH_CNF13_Pos)      /*!< 0x00800000 */


#define DELAY_COUNT 1000000

void delay (unsigned int ii) {
    unsigned int jj;
    for(jj = ii; jj > 0; jj--)
        asm("nop");
}

int main(void){
    // I/O port C and A clock enable
    RCC->APB2ENR = RCC_APB2ENR_IOPCEN | RCC_APB2ENR_IOPAEN;

    // Set PC_13 to output
    GPIOC->CRH &= ~(GPIO_CRH_MODE13 | GPIO_CRH_CNF13);
    GPIOC->CRH |= GPIO_CRH_MODE13;

    while(1) {
        GPIOC->BRR |= 1<<13;
        delay(DELAY_COUNT);
        GPIOC->BSRR |= 1<<13;
        delay(DELAY_COUNT);
    }
}

/* do nothing functions */
int  assert_param     (void *v) { return 0; }
void handle_nmi       (void)    { return;   }
void handle_hardfault (void)    { return;   }

/* vector table */
unsigned int *myvectors[4] __attribute__ ((section("vectors")))= {
    (unsigned int *) STACK_TOP,       // stack pointer
    (unsigned int *) main,            // code entry point
    (unsigned int *) handle_nmi,      // handle non-maskable interrupts
    (unsigned int *) handle_hardfault // handle hard faults
};