情况描述——

4路PWM输出，通过设置占空比，分别是50%，37.5%，25%，12.5%，用万用表观察单片机各个PWM引脚输出电压，分别是3.3V与上述比例相乘的值。

然后，分别将这4路与L298N的ENA端短接起来对比测试L298N输出电压，

发现到了L298N输出端测得的电压就不是这个比例了，除了50%的那路输出将近3V的电压(12V直流减速电机还能转动)，其余各路都接近于0V（电机不转）。




配置代码如下：

/*
* 函数名：TIM3_GPIO_Config
* 描述  ：配置TIM3复用输出PWM时用到的I/O
* 输入  ：无
* 输出  ：无
* 调用  ：内部调用
* 硬件连接：---------------------
*          |  PA.06: (TIM3_CH1)  |
*          |  PA.07: (TIM3_CH2)  |
*              |  PB.00: (TIM3_CH3)  |
*                    |  PB.01: (TIM3_CH4)  |
*           ---------------------   
*/

static void TIM3_GPIO_Config(void)
{
  GPIO_InitTypeDef GPIO_InitStructure;

        /* TIM3 clock enable */
        //PCLK1经过2倍频后作为TIM3的时钟源等于72MHz
  RCC_APB1PeriphClockCmd(RCC_APB1Periph_TIM3, ENABLE);

  /* GPIOA and GPIOB clock enable */
  RCC_APB2PeriphClockCmd(RCC_APB2Periph_GPIOA | RCC_APB2Periph_GPIOB, ENABLE);

  /*GPIOA Configuration: TIM3 channel 1 and 2 as alternate function push-pull */
  GPIO_InitStructure.GPIO_Pin =  GPIO_Pin_6 | GPIO_Pin_7;
  GPIO_InitStructure.GPIO_Mode = GPIO_Mode_AF_PP;                    // 复用推挽输出
  GPIO_InitStructure.GPIO_Speed = GPIO_Speed_50MHz;

  GPIO_Init(GPIOA, &GPIO_InitStructure);

  /*GPIOB Configuration: TIM3 channel 3 and 4 as alternate function push-pull */
  GPIO_InitStructure.GPIO_Pin =  GPIO_Pin_0 | GPIO_Pin_1;

  GPIO_Init(GPIOB, &GPIO_InitStructure);
}

/*
* 函数名：TIM3_Mode_Config
* 描述  ：配置TIM3输出的PWM信号的模式，如周期、极性、占空比
* 输入  ：无
* 输出  ：无
* 调用  ：内部调用
*/
static void TIM3_Mode_Config(void)
{
        TIM_TimeBaseInitTypeDef  TIM_TimeBaseStructure;
        TIM_OCInitTypeDef  TIM_OCInitStructure;

        /* PWM信号电平跳变值 */
        u16 CCR1_Val = 500;        
        u16 CCR2_Val = 375;
        u16 CCR3_Val = 250;
        u16 CCR4_Val = 125;

/* -----------------------------------------------------------------------
    TIM3 Configuration: generate 4 PWM signals with 4 different duty cycles:
    TIM3CLK = 72 MHz, Prescaler = 0x0, TIM3 counter clock = 72 MHz
    TIM3 ARR Register = 999 => TIM3 Frequency = TIM3 counter clock/(ARR + 1)
    TIM3 Frequency = 72 KHz.
    TIM3 Channel1 duty cycle = (TIM3_CCR1/ TIM3_ARR)* 100 = 50%
    TIM3 Channel2 duty cycle = (TIM3_CCR2/ TIM3_ARR)* 100 = 37.5%
    TIM3 Channel3 duty cycle = (TIM3_CCR3/ TIM3_ARR)* 100 = 25%
    TIM3 Channel4 duty cycle = (TIM3_CCR4/ TIM3_ARR)* 100 = 12.5%
  ----------------------------------------------------------------------- */

  /* Time base configuration */                 
  TIM_TimeBaseStructure.TIM_Period = 999;       //当定时器从0计数到999，即为1000次，为一个定时周期
  TIM_TimeBaseStructure.TIM_Prescaler = 0;            //设置预分频：不预分频，即为72MHz
  TIM_TimeBaseStructure.TIM_ClockDivision = TIM_CKD_DIV1 ;        //设置时钟分频系数：不分频
  TIM_TimeBaseStructure.TIM_CounterMode = TIM_CounterMode_Up;  //向上计数模式

  TIM_TimeBaseInit(TIM3, &TIM_TimeBaseStructure);

  /* PWM1 Mode configuration: Channel1 */
  TIM_OCInitStructure.TIM_OCMode = TIM_OCMode_PWM1;            //配置为PWM模式1
  TIM_OCInitStructure.TIM_OutputState = TIM_OutputState_Enable;       
  TIM_OCInitStructure.TIM_Pulse = CCR1_Val;           //设置跳变值，当计数器计数到这个值时，电平发生跳变
  TIM_OCInitStructure.TIM_OCPolarity = TIM_OCPolarity_High;  //当定时器计数值小于CCR1_Val时为高电平

  TIM_OC1Init(TIM3, &TIM_OCInitStructure);         //使能通道1

  TIM_OC1PreloadConfig(TIM3, TIM_OCPreload_Enable);

  /* PWM1 Mode configuration: Channel2 */
  TIM_OCInitStructure.TIM_OutputState = TIM_OutputState_Enable;
  TIM_OCInitStructure.TIM_Pulse = CCR2_Val;          //设置通道2的电平跳变值，输出另外一个占空比的PWM

  TIM_OC2Init(TIM3, &TIM_OCInitStructure);          //使能通道2

  TIM_OC2PreloadConfig(TIM3, TIM_OCPreload_Enable);

  /* PWM1 Mode configuration: Channel3 */
  TIM_OCInitStructure.TIM_OutputState = TIM_OutputState_Enable;
  TIM_OCInitStructure.TIM_Pulse = CCR3_Val;        //设置通道3的电平跳变值，输出另外一个占空比的PWM

  TIM_OC3Init(TIM3, &TIM_OCInitStructure);         //使能通道3

  TIM_OC3PreloadConfig(TIM3, TIM_OCPreload_Enable);

  /* PWM1 Mode configuration: Channel4 */
  TIM_OCInitStructure.TIM_OutputState = TIM_OutputState_Enable;
  TIM_OCInitStructure.TIM_Pulse = CCR4_Val;        //设置通道4的电平跳变值，输出另外一个占空比的PWM

  TIM_OC4Init(TIM3, &TIM_OCInitStructure);        //使能通道4

  TIM_OC4PreloadConfig(TIM3, TIM_OCPreload_Enable);

  TIM_ARRPreloadConfig(TIM3, ENABLE);                         // 使能TIM3重载寄存器ARR

  /* TIM3 enable counter */
  TIM_Cmd(TIM3, ENABLE);                   //使能定时器3       
}

/*
* 函数名：TIM3_PWM_Init
* 描述  ：TIM3 输出PWM信号初始化，只要调用这个函数
*         TIM3的四个通道就会有PWM信号输出
* 输入  ：无
* 输出  ：无
* 调用  ：外部调用
*/
void TIM3_PWM_Init(void)
{
        TIM3_GPIO_Config();
        TIM3_Mode_Config();       
}