CH32V103 systick中断只能进一次如何处理？

初始化

    Systick->CNTL0 =0;
    SysTick->CNTL1 =0;
    SysTick->CNTL2 =0;
    SysTick->CNTL3 =0;
    SysTick->CNTH0 =0;
    SysTick->CNTH1 =0;
    SysTick->CNTH2 =0;
    SysTick->CNTH3 =0;


    i=FSYS/8/4;


    SysTick->CMPLR0 = (u8)(i & 0xFF);
    SysTick->CMPLR1 = (u8)(i >> 8);
    SysTick->CMPLR2 = (u8)(i >> 16);
    SysTick->CMPLR3 = (u8)(i >> 24);
    SysTick->CMPHR0 =0;
    SysTick->CMPHR1 =0;
    SysTick->CMPHR2 =0;
    SysTick->CMPHR3 =0;
    SysTick->CTLR=1;
   NVIC_EnableIRQ(SysTicK_IRQn);


中断程序翻PB8闪灯:
    GPIOB->OUTDR^=1<<8;
    SysTick->CNTL0 =0;

    SysTick->CNTL1 =0;
    SysTick->CNTL2 =0;
    SysTick->CNTL3 =0;
    SysTick->CNTH0 =0;
    SysTick->CNTH1 =0;
    SysTick->CNTH2 =0;
    SysTick->CNTH3 =0;
单独这样跑没有问题,
随便增加USART1发送一字节,就不闪了,CPU没有死,读systick值,CNT在增加,就是不进中断