皮质m3上的systick示例不起作用

Question

我正在尝试测量功能期间经过的时间，但我不确定我是否正确执行此操作。当我停止systick时，它返回的值是24，这是不可能的。

#define SYSTICKS 0xFFFFFFu
static volatile  uint32_t Count = 0;

void SysTick_Handler(void)
{
    Count++;
}

int doSomething()
{
    int a = 0;
    int b = 8;

    return a + b;
}
int main(int argc, char *argv[])

    SysTick_Config(SYSTICKS);

    volatile uint64_t a = (uint64_t)SYSTICKS - SysTick->VAL;
    a += ((uint64_t)SYSTICKS * Count);

    /* some code here */
    doSomething();

    volatile uint64_t b = (uint64_t)SYSTICKS - SysTick->VAL;
    b += ((uint64_t)SYSTICKS * Count);

    volatile uint64_t timeElapsed = a - b;
    printf("a = %lld\r\n", a);
    printf("b = %lld\r\n", b);
    printf("timeElapsed = %lld\n", timeElapsed);

这是输出

a = 16777215
b = 24
timeElapsed = 16777191