STM32比较输出PMW频率改变的问题

因为需要控制步进电机的转速，所以要实时改变PMW的频率。我用输出比较的方法输出一路PMW波形。具体程序如下 ：

tiM4配置：

void TIM4_Configuration(void)
{
        GPIO_InitTypeDef GPIO_InitStructure;
        TIM_OCInitTypeDef  TIM_OCInitStructure;
        TIM_TimeBaseInitTypeDef  TIM_TimeBaseStructure;
        NVIC_InitTypeDef NVIC_InitStructure;


        RCC_APB1PeriphClockCmd(RCC_APB1Periph_TIM4, ENABLE);
        RCC_APB2PeriphClockCmd(RCC_APB2Periph_GPIOB, ENABLE);

        GPIO_InitStructure.GPIO_Pin =  GPIO_Pin_7|GPIO_Pin_8|   

GPIO_Pin_9|GPIO_Pin_6;
    GPIO_InitStructure.GPIO_Mode = GPIO_Mode_AF_PP;
    GPIO_InitStructure.GPIO_Speed = GPIO_Speed_50MHz;
    GPIO_Init(GPIOB, &GPIO_InitStructure);

        NVIC_PriorityGroupConfig(NVIC_PriorityGroup_1);
    NVIC_InitStructure.NVIC_IRQChannel = TIM4_IRQn;
    NVIC_InitStructure.NVIC_IRQChannelPreemptionPriority = 0;
    NVIC_InitStructure.NVIC_IRQChannelSubPriority = 1;
    NVIC_InitStructure.NVIC_IRQChannelCmd = ENABLE;
    NVIC_Init(&NVIC_InitStructure);

        TIM_TimeBaseStructure.TIM_Period = 65535;
    TIM_TimeBaseStructure.TIM_Prescaler = 29;
    TIM_TimeBaseStructure.TIM_ClockDivision = 0;
    TIM_TimeBaseStructure.TIM_CounterMode = TIM_CounterMode_Up;
    TIM_TimeBaseInit(TIM4, &TIM_TimeBaseStructure);




    TIM_OCInitStructure.TIM_OCMode = TIM_OCMode_Toggle;
    TIM_OCInitStructure.TIM_OutputState =     

    TIM_OutputState_Enable;
    TIM_OCInitStructure.TIM_Pulse = 240;
    TIM_OCInitStructure.TIM_OCPolarity = TIM_OCPolarity_Low;
    TIM_OC1Init(TIM4, &TIM_OCInitStructure);
    TIM_OC1PreloadConfig(TIM4, TIM_OCPreload_Disable);


    TIM_ARRPreloadConfig(TIM4, DISABLE);

    TIM_Cmd(TIM4, ENABLE);

    TIM_ITConfig(TIM4,TIM_IT_CC1|TIM_IT_CC2|TIM_IT_CC3|   

TIM_IT_CC4, ENABLE);

}


TIM4中断函数：

void TIM4_IRQHandler(void)
{
  u16 capture ;

  if (TIM_GetITStatus(TIM4, TIM_IT_CC1) != RESET)
   {
      TIM_ClearITPendingBit(TIM4, TIM_IT_CC1);

          capture = TIM_GetCapture1(TIM4);

          TIM4_Aim_Counter1 = (CmdCode[2] << 8) | CmdCode[3];

          TIM4_Aim_Counter2 = (CmdCode[4] << 8) | CmdCode[5];

          TIM_SetCompare1(TIM4,capture + TIM4_Aim_Counter1);        

          TIM4_CH4_Counter++;

          if(TIM4_CH4_Counter >= TIM4_Aim_Counter2 )        
          {
              TIM_Cmd(TIM4, DISABLE);

                  TIM4_CH4_Counter = 0;

                    for(i = 0;i < 10;i++)
                        {
                                CmdCode[i] = 0;
                        }                 

          }  
   }
}

程序如上面所示。我之前做过实验，通过改变TIM4_Aim_Counter1的来改变PMW的输出频率，并且成功的，波形的占空比都是50%

比如TIM4_Aim_Counter1的值为150，输出PMW频率为8KHz,TIM4_Aim_Counter1的值为300，输出PMW频率为4KHz.但是很多人说改变这个TIM4_Aim_Counter1只能改变占空比不能改频率。但是我这个怎么能改呢？如果我想实时的改变PMW的输出频率，改怎么改呢？
如下图所示。