stm32定时器3的PWM无输出程序是为什么？

    #include "pwm_output.h"
void tiM3_PWM_Init(void)
{
TIM3_GPIO_Config();
TIM3_Mode_Config();

}
static void TIM3_GPIO_Config()
{
GPIO_InitTypeDef GPIO_InitStructure;

RCC_APB2PeriphClockCmd(RCC_APB2Periph_GPIOA|RCC_APB2Periph_GPIOB,ENABLE);
RCC_APB1PeriphClockCmd(RCC_APB1Periph_TIM3,ENABLE);

GPIO_InitStructure.GPIO_Mode =GPIO_Mode_AF_PP;
GPIO_InitStructure.GPIO_Pin =GPIO_Pin_6|GPIO_Pin_7;
GPIO_InitStructure.GPIO_Speed =GPIO_Speed_50MHz;
GPIO_Init(GPIOA, GPIO_InitStructure);

GPIO_InitStructure.GPIO_Pin =GPIO_Pin_0|GPIO_Pin_1;
GPIO_Init(GPIOB, GPIO_InitStructure);

}
static void TIM3_Mode_Config()
{
TIM_TimeBaseInitTypeDef TIM_TimeBaseInitStructure;
TIM_OCInitTypeDef TIM_OCInitStructure;
u16 CCR1_Val=750;
u16 CCR2_Val=500;
u16 CCR3_Val=250;
u16 CCR4_Val=100;

TIM_DeInit(TIM3);
TIM_TimeBaseInitStructure.TIM_ClockDivision =TIM_CKD_DIV1;
TIM_TimeBaseInitStructure.TIM_CounterMode =TIM_CounterMode_Up;
TIM_TimeBaseInitStructure.TIM_Period =999;
TIM_TimeBaseInitStructure.TIM_Prescaler =0;
TIM_TimeBaseInit(TIM3, TIM_TimeBaseInitStructure);

TIM_OCInitStructure.TIM_OCMode =TIM_OCMode_PWM1;
TIM_OCInitStructure.TIM_OCPolarity =TIM_OCPolarity_High;
TIM_OCInitStructure.TIM_OutputState =TIM_OutputState_Enable;
TIM_OCInitStructure.TIM_Pulse =CCR1_Val;
TIM_OC1Init(TIM3, TIM_OCInitStructure);
TIM_OC1PreloadConfig(TIM3,TIM_OCPreload_Enable);

TIM_OCInitStructure.TIM_OutputState =TIM_OutputState_Enable;
TIM_OCInitStructure.TIM_Pulse =CCR2_Val;
TIM_OC2Init(TIM3, TIM_OCInitStructure);
TIM_OC2PreloadConfig(TIM3,TIM_OCPreload_Enable);

TIM_OCInitStructure.TIM_OutputState =TIM_OutputState_Enable;
TIM_OCInitStructure.TIM_Pulse =CCR3_Val;
TIM_OC3Init(TIM3, TIM_OCInitStructure);
TIM_OC3PreloadConfig(TIM3,TIM_OCPreload_Enable);

TIM_OCInitStructure.TIM_OutputState =TIM_OutputState_Enable;
TIM_OCInitStructure.TIM_Pulse =CCR4_Val;
TIM_OC4Init(TIM3, TIM_OCInitStructure);
TIM_OC4PreloadConfig(TIM3,TIM_OCPreload_Enable);

TIM_Cmd(TIM3,ENABLE);

}