我用STM32想要生成50Hz的PWM波，但是不管怎么设置PWM参数。用简易示波器观察频率一直是1KHz，但是占空比是可以被设置改变的，求各位老哥给把把脉，附PWM配置
#include "pwm.h"

void pwm_init(void)
{
  pwm_gpio_init();

  TIM_OCInitTypeDef TIM_OCInitStructure;
  TIM_TimeBaseInitTypeDef TIM_TimeBaseStructure;

  RCC_APB1PeriphClockCmd(RCC_APB1Periph_TIM3, ENABLE);

  TIM_TimeBaseStructure.TIM_Period = (20000-1);   //不管怎么改period和prescaler这两个参数，频率一直是1KHz
  TIM_TimeBaseStructure.TIM_Prescaler = (72-1);
  TIM_TimeBaseStructure.TIM_ClockDivision = TIM_CKD_DIV1;
  TIM_TimeBaseStructure.TIM_CounterMode = TIM_CounterMode_Up;
  TIM_TimeBaseInit(TIM2, & TIM_TimeBaseStructure);

  TIM_OCInitStructure.TIM_OCMode = TIM_OCMode_PWM1;
  TIM_OCInitStructure.TIM_Pulse = 0x0000;
  TIM_OCInitStructure.TIM_OutputState = TIM_OutputState_Enable;
  TIM_OCInitStructure.TIM_OCPolarity = TIM_OCPolarity_High;

  TIM_OC1Init(TIM3, & TIM_OCInitStructure);
  TIM_OC2Init(TIM3, & TIM_OCInitStructure);

  TIM_OC1PreloadConfig(TIM3, TIM_OCPreload_Enable);
  TIM_OC2PreloadConfig(TIM3, TIM_OCPreload_Enable);

  TIM_ARRPreloadConfig(TIM3, ENABLE);
  TIM_Cmd(TIM3, ENABLE);

}

void pwm_gpio_init()
{
  GPIO_InitTypeDef GPIO_InitStructure;

  RCC_APB2PeriphClockCmd(RCC_APB2Periph_GPIOA |RCC_APB2Periph_AFIO , ENABLE);

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