设置的PWM频率应该是输出50赫兹，但是输出却是5赫兹，求解，通用定时器初始化频率是72M。

void PWM_iNIT(void)
{
GPIO_InitTypeDef GPIO_InitStructure;
TIM_TimeBaseInitTypeDef Tim_Initstructure;
TIM_OCInitTypeDef TIM_OCInitStructure;

RCC_APB1PeriphClockCmd(RCC_APB1Periph_TIM2,ENABLE);

RCC_APB2PeriphClockCmd(RCC_APB2Periph_GPIOA,ENABLE);

// TIM_InternalClockConfig(TIM2);

GPIO_InitStructure.GPIO_Pin = GPIO_Pin_1;
GPIO_InitStructure.GPIO_Mode = GPIO_Mode_AF_PP; //复用推挽，使输出控制权来自定时器而不是输出数据寄存器
GPIO_InitStructure.GPIO_Speed = GPIO_Speed_50MHz;
GPIO_Init(GPIOA, &GPIO_InitStructure);

Tim_Initstructure.TIM_ClockDivision = TIM_CKD_DIV1;
Tim_Initstructure.TIM_CounterMode = TIM_CounterMode_Up;
Tim_Initstructure.TIM_Period = 19999;
Tim_Initstructure.TIM_Prescaler = 71;
Tim_Initstructure.TIM_RepetitionCounter = 0;
TIM_TimeBaseInit(TIM2,&Tim_Initstructure);

TIM_OCStructInit(&TIM_OCInitStructure); //给结构体赋初始值，后面吧要改的值赋值就行，就不用给结构体全体成员赋值
TIM_OCInitStructure.TIM_OCMode=TIM_OCMode_PWM1;
TIM_OCInitStructure.TIM_OCPolarity=TIM_OCPolarity_High; //比较输出极性
TIM_OCInitStructure.TIM_OutputState=TIM_OutputState_Enable; //输出使能
TIM_OCInitStructure.TIM_Pulse=0;
TIM_OC2Init(TIM2,&TIM_OCInitStructure);
TIM_OC2PreloadConfig(TIM2, TIM_OCPreload_Enable);
TIM_ARRPreloadConfig(TIM2,ENABLE);
TIM_Cmd(TIM2,ENABLE);

}