TIM8的CH2N输出周期不对

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        设置TIM8的频率为800kHz，但是用示波器看实际输出的波形频率只有50Hz，有同志能指点一下吗？
        TIM_TimeBaseStructure.TIM_Period = 90-1; // 800kHz
        TIM_TimeBaseStructure.TIM_Prescaler = 0;
        TIM_TimeBaseStructure.TIM_ClockDivision = 0;
        TIM_TimeBaseStructure.TIM_CounterMode = TIM_CounterMode_Up;
        TIM_TimeBaseInit(TIM8, &TIM_TimeBaseStructure);

         TIM_OCInitStructure.TIM_OCMode = TIM_OCMode_PWM2; //      
        TIM_OCInitStructure.TIM_OutputNState = TIM_OutputNState_Enable; //
        TIM_OCInitStructure.TIM_Pulse = 0;
        TIM_OCInitStructure.TIM_OCNPolarity = TIM_OCNPolarity_Low; //
        TIM_OCInitStructure.TIM_OCNIdleState = TIM_OCNIdleState_Reset;
        TIM_OC2Init(TIM8, &TIM_OCInitStructure);  //
        TIM_OC2PreloadConfig(TIM8, TIM_OCPreload_Enable); //
        TIM_ARRPreloadConfig(TIM8, ENABLE);
        TIM_SetCompare2(TIM8,45);

        TIM_CtrlPWMOutputs(TIM8,ENABLE);
        TIM_Cmd(TIM8, ENABLE);