ʹCMP1,PʹGPIO루PA11NʹGPIO루PA7
 ʩ״̬
 2.5V             -> PA11
   0V              -> PA7
 PA4 -> HIGH 


ڱԴѡTIM8OC4
TIM8ͨ4PD1PWMΣΪߵƽʱPA4 -> LOW. 