定时器1 PWM输出为什么会存在延迟

RCC_APB2PeriphClockCmd(RCC_APB2Periph_TIM1, ENABLE);        

        //²»·ÖƵ¡£PWMƵÂÊ=72000000/900=80Khz

        /* ---------------------------------------------------------------

        TIM3CLK ¼´PCLK1=36MHz

        TIM3 Configuration: generate 1 PWM signals :

    TIM3CLK = 36 MHz, Prescaler = 0x0, TIM3 counter clock = 36 MHz

    TIM3 ARR Register = 900 => TIM3 Frequency = TIM3 counter clock/(ARR + 1)

    TIM3 Frequency = 36 KHz.

    TIM3 Channel2 duty cycle = (TIM3_CCR2/ TIM3_ARR)* 100 

        TIM3CLK = 36 MHz, Prescaler = 0, TIM3 counter clock = 36MHz

        --------------------------------------------------------------- */

        /* Time base configuration */

        TIM_TimeBaseStructure.TIM_Period = 20000-1; //ÉèÖÃÔÚÏÂÒ»¸ö¸üÐÂʼþ×°Èë»î¶¯µÄ×Ô¶¯ÖØ×°ÔؼĴæÆ÷ÖÜÆÚµÄÖµ         50Hz

        TIM_TimeBaseStructure.TIM_Prescaler =72-1; //ÉèÖÃÓÃÀ´×÷ΪTIMxʱÖÓƵÂʳýÊýµÄÔ¤·ÖƵֵ  ²»·ÖƵ

        TIM_TimeBaseStructure.TIM_ClockDivision = TIM_CKD_DIV2; //ÉèÖÃʱÖÓ·Ö¸î:TDTS = Tck_tim

        TIM_TimeBaseStructure.TIM_CounterMode = TIM_CounterMode_Up;  //TIMÏòÉϼÆÊýģʽ

        TIM_TimeBaseInit(TIM1, &TIM_TimeBaseStructure); //¸ù¾ÝTIM_TimeBaseInitStructÖÐÖ¸¶¨µÄ²ÎÊý³õʼ»¯TIMxµÄʱ¼ä»ùÊýµ¥Î»

        

        /* Output Compare Active Mode configuration: Channel1 */

        TIM_OCInitStructure.TIM_OCMode = TIM_OCMode_PWM1; //Ñ¡Ôñ¶¨Ê±Æ÷ģʽ:TIMÂö³å¿í¶Èµ÷ÖÆģʽ2

        TIM_OCInitStructure.TIM_OutputState = TIM_OutputState_Disable; //±È½ÏÊä³öʹÄÜ

        TIM_OCInitStructure.TIM_OutputNState = TIM_OutputNState_Enable; 

        TIM_OCInitStructure.TIM_Pulse = 0; //ÉèÖôý×°È벶»ñ±È½Ï¼Ä´æÆ÷µÄÂö³åÖµ

        TIM_OCInitStructure.TIM_OCNPolarity = TIM_OCNPolarity_High;     //??????  

         TIM_OCInitStructure.TIM_OCPolarity = TIM_OCPolarity_High;     //??????  

 

        TIM_OC1Init(TIM1, &TIM_OCInitStructure);  //¸ù¾ÝTIM_OCInitStructÖÐÖ¸¶¨µÄ²ÎÊý³õʼ»¯ÍâÉèTIMx

        TIM_OC1PreloadConfig(TIM1, TIM_OCPreload_Enable);  //ʹÄÜTIMxÔÚCCR3ÉϵÄԤװÔؼĴæÆ÷

        TIM_OC2Init(TIM1, &TIM_OCInitStructure);  //¸ù¾ÝTIM_OCInitStructÖÐÖ¸¶¨µÄ²ÎÊý³õʼ»¯ÍâÉèTIMx

        TIM_OC2PreloadConfig(TIM1, TIM_OCPreload_Enable);  //ʹÄÜTIMxÔÚCCR3ÉϵÄԤװÔؼĴæÆ÷

        TIM_OC3Init(TIM1, &TIM_OCInitStructure);  //¸ù¾ÝTIM_OCInitStructÖÐÖ¸¶¨µÄ²ÎÊý³õʼ»¯ÍâÉèTIMx

        TIM_OC3PreloadConfig(TIM1, TIM_OCPreload_Enable);  //ʹÄÜTIMxÔÚCCR3ÉϵÄԤװÔؼĴæÆ÷

        

        

        TIM_ARRPreloadConfig(TIM1, ENABLE); //ʹÄÜTIMxÔÚARRÉϵÄԤװÔؼĴæÆ÷

        TIM_CtrlPWMOutputs(TIM1,ENABLE);

        /* TIM3 enable counter */

        TIM_Cmd(TIM1, ENABLE);  //ʹÄÜTIMxÍâÉè

int main(void)

{

  //u16 k=0;

        RCC_Configuration();

  GPIO_Configuration();

  PWM_Configuration();         

  delay_init(72);

        //USART2_Config();           //³õʼ»¯ ÅäÖÃUSART2 £¨ÓÃÓÚ½ÓÊÕMPU6050Êý¾Ý£©         

        //nRF24L01_Configuration();

        

        

#ifdef OLED

        OLED_Init(); 

#endif 

        /*   Æô¶¯Ç°ÏµÍ³²âÊÔ     */                

  while(1)

        {

        delay_ms(1000);

        

        GPIO_WriteBit(GPIOC, GPIO_Pin_13, 1);

        TIM_SetCompare3(TIM1,1500);

        

        delay_ms(1000);

        

        

        GPIO_WriteBit(GPIOC, GPIO_Pin_13, 0);

        TIM_SetCompare3(TIM1,2800);



        }

}