| 使用核内部timer定时器实现延时函数,为什么系统时钟要除以4? 
 
 void delay_1ms(uint32_t count)
{
   uint64_t start_mtime, delta_mtime;
   uint64_t tmp = get_timer_value();
   do{
        start_mtime = get_timer_value();
   }while(start_mtime == tmp);
  do{
        delta_mtime = get_timer_value() - start_mtime;
   }while(delta_mtime <(SystemCoreClock/4000 * count));
   
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