TIM_TimeBaseInitTypeDef TIM_TimeBaseStructure;
TIM_TimeBaseStructure.TIM_Period = 0xFFFF;    
TIM_TimeBaseStructure.TIM_Prescaler = 36231;  
TIM_TimeBaseStructure.TIM_ClockDivision = TIM_CKD_DIV2;
TIM_TimeBaseStructure.TIM_CounterMode = TIM_CounterMode_Up;
TIM_TimeBaseInit(TIM7, &TIM_TimeBaseStructure);
TIM_Cmd(TIM7,ENABLE);

static inline uint16_t GetTimerCount(TIM_TypeDef *timer)
{
  return timer->CNT;
}

void SYSTIMER_Set(TIM_TypeDef *timer, uint16_t *compare_val, uint32_t count_ms)
{
    uint16_t timer_count_val = GetTimerCount(timer);
    *compare_val = timer_count_val + count_ms;
}

uint32_t SYSTIMER_Elapsed(TIM_TypeDef *timer, uint16_t *compare_val)
{
    uint16_t timer_count_val = GetTimerCount(timer);
    return (timer_count_val >= *compare_val);
}

while(1)
{
    if (SYSTIMER_Elapsed(TIM7, &compare_val))
    {
         //do something every 100ms
         SYSTIMER_Set(TIM7, &compare_v, 100);
    }
}

uint16_t var = 0Xfff0;
var += 0Xff;
printf("var is %x /n", var);

void delay_1us (void) {
  uint32_t start, end;
  
  start = DWT_CYCCNT;
  end = start + HCLK/1000000;
  if (end < start) while (dwt_cnt() > start);
  while (end >= dwt_cnt());
}

uint32_t DWT_Get(void)
{
    return DWT_CYCCNT;
}

uint8_t DWT_Compare(int32_t tp)
{
    return (((int32_t)DWT_Get() - tp) < 0);
}

void DWT_Delay(uint32_t us) // microseconds
{
    int32_t tp = DWT_Get() + us * (SystemCoreClock/1000000));
    while (DWT_Compare(tp));
}