使能TIM8 AOE=1,CMP_P(PC0)接PB3, COMP1_N(PC1)接VCC/2，COMP1_O(PA4)高电平清除OCXREF

/*
  TIM8:
  CH1  :PD3_AF1
  CH1N :PD0_AF5
  CH2  :PD2_AF5
  CH2N :PC7_AF5
  CH3  :PD1_AF5 
  CH3N :PC6_AF1
  CH4  :PC5_AF1 
  CH5  :PB1_AF1
  CH6  :PB2_AF1
  ETR  :PC3_AF1
  BKIN :PC5_AF5
  BKIN2:PD5_AF3
  */ 