//PrescalerValue = (uint16_t) ((SystemCoreClock /2) / 21000000) - 1;

  /* Time base configuration */
  TIM_TimeBaseStructure.TIM_Period = pre_val;
  TIM_TimeBaseStructure.TIM_Prescaler = psc_val;
  TIM_TimeBaseStructure.TIM_ClockDivision = 0;
  TIM_TimeBaseStructure.TIM_CounterMode = TIM_CounterMode_Up;
  TIM_TimeBaseInit(TIM4, &TIM_TimeBaseStructure);

  /* PWM1 Mode configuration: Channel1 */
  TIM_OCInitStructure.TIM_OCMode = TIM_OCMode_PWM1;
  TIM_OCInitStructure.TIM_OutputState = TIM_OutputState_Enable;
  TIM_OCInitStructure.TIM_Pulse = pre_val/2;
  TIM_OCInitStructure.TIM_OCPolarity = TIM_OCPolarity_High;
  TIM_OC1Init(TIM4, &TIM_OCInitStructure);

  TIM_ARRPreloadConfig(TIM4, ENABLE);
  TIM_Cmd(TIM4, ENABLE);
 
  TIM_ITConfig(TIM4, TIM_IT_Update, ENABLE);
  NVIC_EnableIRQ(TIM4_IRQn);