TIM2_CH1和TIM2_CH2是一组,轮流交替生成SPWM,TIM2_CH1生成SPWM时,TIM2_CH2保持为0;TIM2_CH2生成SPWM时,TIM2_CH1保持为0; TIM2_CH3和TIM2_CH4是一组,轮流交替生成SPWM,TIM2_CH3生成SPWM时,TIM2_CH4保持为0;TIM2_CH4生成SPWM时,TIM2_CH3保持为0; PWM频率40K, 要求在TIM2_CH1的SPWM中点时,TIM2_CH3开始生成SPWM,(两组波形的相位相差90度)