#define USE_STDPERIPH_DRIVER
#include "stm32_p103.h"

void SysTick_Handler(void) {
    GPIOC->ODR ^= 0x00001000;
}

int main(void) {
    int last_button_state, new_button_state;
    uint32_t hclk_ticks_per_sec, ext_clock_ticks_per_sec;

    init_led();
    init_button();

    /* The Cortex System Timer (SysTick) clock can be driven by the Cortex
     * clock (HCLK), which is the output of the AHB prescaler, or from the
     * external clock, which is HLCK divided by 8.
     * Call SysTick_Config with the number of ticks between Systick interrupts.
     * We will choose this value such that there is one second
     * between interrupts when the external clock (HCLK / 8) is selected.
     * Initially, the Cortex clock (HCLK) will be selected, which means an
     * interrupt every 1/8th of a second.
     */
    hclk_ticks_per_sec = SystemCoreClock;
    ext_clock_ticks_per_sec = hclk_ticks_per_sec / 8;
    if (SysTick_Config(ext_clock_ticks_per_sec)) {
        /* If SysTick_Config returns 1, that means the number ticks exceeds the
         * limit. */
        while (1)
            ;
    }

    /* Infinite loop - when the button changes state, toggle the SysTick clock
     * source.
     */
    last_button_state = GPIOA->IDR & 0x00000001;
    while (1) {
        SysTick->CTRL ^= SysTick_CTRL_CLKSOURCE_Msk;
        SysTick->VAL;
    }
}