RCC->AHBENR |= RCC_AHBENR_GPIOAEN;
  GPIOA->MODER |= GPIO_MODER_MODER7_1;
  GPIOA->PUPDR |= GPIO_PUPDR_PUPDR7_1;
  GPIOA->AFR[0] |= 0x1<<GPIO_AFRL_AFSEL7_Pos;//alternative mode tim3
  RCC->APB1ENR |= RCC_APB1ENR_TIM3EN;
  TIM3->PSC = 3;
  TIM3->SMCR |= TIM_SMCR_TS_2 | TIM_SMCR_TS_1 | TIM_SMCR_SMS_2;
  TIM3->CCMR1 |= TIM_CCMR1_CC2S_0 | TIM_CCMR1_CC1S_1;
  TIM3->CCER |= TIM_CCER_CC1E | TIM_CCER_CC2E | TIM_CCER_CC1P;
  TIM3->DIER |= TIM_DIER_CC2IE;
  TIM3->CR1 |= TIM_CR1_CEN;
  NVIC_SetPriority (TIM3_IRQn, 0);
  NVIC_EnableIRQ(TIM3_IRQn);