基于STM32四轴飞行器用串口助手控制电机开关反应不理想

具体情况就是，在串口助手给了1个00或者01电机反应不理想，电机不是单纯的开或者关，而是停留在刚给00或者01这个转速的瞬间，即是给了00或者01，电机转速不变了。以下是我的中断服务函数的代码和MAIN函数的代码。
//中断服务函数：
void USART1_IRQHandler(void)   //
{
u16 a;
while(1)
{
while(USART_GetFlagStatus(USART1, USART_FLAG_RXNE) == RESET){}
  //if(USART_GetiTStatus(USART2, USART_IT_RXNE) != RESET)
a= USART_ReceiveData(USART1);
  switch(a)    /
{
case 0:a2=0;break;
case 1:a2=1;break;  
};
USART_SendData(USART1,a2);
while(USART_GetFlagStatus(USART1, USART_FLAG_RXNE) == RESET){}
//USART_RX_STA=0;
}
}
//MAIN函数：
int a2=0;
int a3=100;
int main(void)
{
//u16 led0pwmval=0;   
//u8 dir=1;
//u8 t=0;
  int b=1;
//KEY_Init();          //????????°??ü??????????????
NVIC_Configuration();
delay_init();     //???±??????????  
uart_init(9600); //?®??????????9600
  delay_init();     //???±??????????  
LED_Init();  //????????LED??????????????

  TIM3_PWM_Init(200,720);//??·?????PWM????=72000/(899+1)=80Khz   
  delay_ms(1500);
delay_ms(1500);
delay_ms(1500);
  TIM_SetCompare1(TIM3,100);  
TIM_SetCompare2(TIM3,100);
TIM_SetCompare3(TIM3,100);
TIM_SetCompare4(TIM3,100);
while(1)
{
if(a2==0)
delay_ms(50);
if(b)a3++;
else a3--;
if(a3>199) b=0;
if(a3==100)  b=1;
TIM_SetCompare1(TIM3,a3);
TIM_SetCompare2(TIM3,a3);
TIM_SetCompare3(TIM3,a3);
TIM_SetCompare4(TIM3,a3);
if(a2==1)



TIM_SetCompare1(TIM3,0);
TIM_SetCompare2(TIM3,0);
TIM_SetCompare3(TIM3,0);
TIM_SetCompare4(TIM3,0);
}
}