各位为什么我用stm32只能的通用计数器只能输出三路pwm波？

在软件能出来思路波形，但我示波器却只能观察到三路。一下是是pwm波的程序，麻烦各位看看，帮忙找找是哪里出错了？？/**  ******************************************************************************
  * @file    bsp_pwm_output.c
  * @author  STMicroelectronics
  * @version V1.0
  * @date    2013-xx-xx
  * @brief   tim3输出四路pwm bsp
  ******************************************************************************
  * @attention
  *
  *
  ******************************************************************************
  */

#include "bsp_pwm_output.h"

/**
  * @brief  配置TIM3复用输出PWM时用到的I/O
  * @param  无
  * @retval 无
  */
static void TIM3_GPIO_Config(void)
{
  GPIO_InitTypeDef GPIO_InitStructure;

        /* 设置TIM3CLK 为 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);
}

/**
  * @brief  配置TIM3输出的PWM信号的模式，如周期、极性、占空比
  * @param  无
  * @retval 无
  */
/*
* TIMxCLK/CK_PSC --> TIMxCNT --> TIMx_ARR --> TIMxCNT 重新计数
*                    TIMx_CCR(电平发生变化)
* 信号周期=(TIMx_ARR +1 ) * 时钟周期
* 占空比=TIMx_CCR/(TIMx_ARR +1)
*/
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 = 500;
        u16 CCR3_Val = 500;
        u16 CCR4_Val = 500;

/* -----------------------------------------------------------------------
    TIM3 Channel1 duty cycle = (TIM3_CCR1/ TIM3_ARR+1)* 100% = 50%
    TIM3 Channel2 duty cycle = (TIM3_CCR2/ TIM3_ARR+1)* 100% = 37.5%
    TIM3 Channel3 duty cycle = (TIM3_CCR3/ TIM3_ARR+1)* 100% = 25%
    TIM3 Channel4 duty cycle = (TIM3_CCR4/ TIM3_ARR+1)* 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(这里用不到)
  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       
}