TMS320F28377S-Q1: EPWM计数器模式切换

EPWM配置为增计数模式并配置动作寄存器EPwm6Regs.AQCTLA.bit.ZRO = AQ_SET;EPwm6Regs.AQCTLA.bit.CAU = AQ_CLEAR;运行一段时间切换成减计数模式，并配置动作寄存器EPwm6Regs.AQCTLA.bit.ZRO = AQ_CLEAR;EPwm6Regs.AQCTLA.bit.CAD = AQ_SET;

遇到问题：切换临界点的那个开关周期，无法使计数模式立刻改变，再下一个周期就正常了。我想问一下为什么会这样？是否能够通过配置避免这个问题？

代码如下：

__interrupt void epwm6_isr(void)

{

++EPWMFLAG;
if(EPWMFLAG<50)
{
EPwm6Regs.TBCTL.bit.CTRMODE = TB_COUNT_UP;
EPwm6Regs.AQCTLA.bit.ZRO = AQ_SET;
EPwm6Regs.AQCTLA.bit.CAU = AQ_CLEAR;
}
else if(EPWMFLAG>=50&&EPWMFLAG<100)
{
EPwm6Regs.TBCTL.bit.CTRMODE = TB_COUNT_DOWN;
EPwm6Regs.AQCTLA.bit.ZRO = AQ_CLEAR;
EPwm6Regs.AQCTLA.bit.CAD = AQ_SET;

}
else
{
EPWMFLAG=0;

}

EPwm6Regs.ETCLR.bit.INT = 1;
PieCtrlRegs.PIEACK.all = PIEACK_GROUP3;

}