TBPRD幅值后，会变为0？？？

EALLOW;
SysCtrlRegs.PCLKCR0.bit.TBCLKSYNC = 0; // Stop all the TB clocks
EDIS;
// Setup Sync
EPwm1Regs.TBCTL.bit.SYNCOSEL = TB_SYNC_IN; // Pass through

//********************epwm1 config*********************//
EPwm1Regs.TBCTL.bit.PHSEN = TB_ENABLE; //Start
EPwm1Regs.TBPHS.half.TBPHS = 0; //
EPwm1Regs.TBPRD = PWM1_TIMER_TBPRD; //
EPwm1Regs.CMPA.half.CMPA = PWM1_TIMER_TBPRD/2;
/* Init Timer-Base Control Register for EPWM1-EPWM3*/ \
// EPwm1Regs.TBCTL.all = 0xA00A;
EPwm1Regs.TBCTL.bit.CTRMODE=0x2;
EPwm1Regs.TBCTL.bit.PRDLD =0x1;
EPwm1Regs.TBCTL.bit.PHSEN = 0x0; //Start

请教各位大神，为什么我在执行EPwm1Regs.TBCTL.bit.PRDLD =0x1;发现TBPRD寄存器内的值居然变成了0！！！！