PWM周期偶尔会变化问题

如题，

设置20Khz PWM开关频率，未修改TBPRD的情况下，

测试PWM偶尔会出现PWM开关频率不是20Khz的问题，有时偏大，有时偏小，不是每次都出。

配置TBCLK = SYSCLKOUT/(HSPCLKDIV*CLKDIV)，中HSPCLKDIV，CLKDIV = 1；即TBCLK=SYSCLKOUT，是否是这个原因？

PWM配置如下：

void InitEPwm1(void)
{
// 时基寄存器初始化
EPwm1Regs.TBPRD = SysConfig.PWM_Half_Period;                          // Set timer period=HSPCLK/fs/2=75M/1.6K/2=1875
EPwm1Regs.TBPHS.half.TBPHS = 0;                                      // Set timer Phase is 0

EPwm1Regs.TBCTR = 0x0000;                                              // Clear counter

EPwm1Regs.TBCTL.bit.FREE_SOFT = 0x3;                       //free run
EPwm1Regs.TBCTL.bit.PHSDIR = 0x0;                            // don't care
EPwm1Regs.TBCTL.bit.CLKDIV = 0x0;                             // prescaler = 1
EPwm1Regs.TBCTL.bit.HSPCLKDIV = 0x0;                    // prescaler = 1, TBCLK = SYSCLKOUT/(HSPCLKDIV*CLKDIV)=75MHz
EPwm1Regs.TBCTL.bit.SWFSYNC = 0x0;
EPwm1Regs.TBCTL.bit.SYNCOSEL = 0x1;                    // 00 Sync output From SYNCI||01 Sync output when TBCTR=0
EPwm1Regs.TBCTL.bit.PRDLD = 0x0;                            // TBPRD reload from shadow
EPwm1Regs.TBCTL.bit.PHSEN = 0x0;                            // Master module, Disable phase loading
EPwm1Regs.TBCTL.bit.CTRMODE = 0x2;                      // Count up and down

// 比较寄存器初始化
EPwm1Regs.CMPCTL.bit.SHDWAMODE = 0x0;
EPwm1Regs.CMPCTL.bit.LOADAMODE = 0x2; // Reload CMPA/CMPB When TBCTR=0 or TBPRD

// 动作确认寄存器初始化
EPwm1Regs.AQCTLA.all = 0;

// PWMxA and PWMxB interlace 180 degree
EPwm1Regs.AQCTLA.bit.CAU = 0x2;               // clear PWMxA on up = CMPA
EPwm1Regs.AQCTLA.bit.CAD = 0x1;               // set PWMxA on down = CMPA
EPwm1Regs.AQCTLA.bit.PRD = 0x0;                 // CNT=Zero or PRD no action
EPwm1Regs.AQCTLA.bit.ZRO = 0x0;

// 死区寄存器初始化(DSP内部死区为低)
EPwm1Regs.DBCTL.bit.IN_MODE = 0x0;                 // IN_mode selection
EPwm1Regs.DBCTL.bit.POLSEL = 0x2;                   // Active high complementary
EPwm1Regs.DBRED = DeadBand_1P5us;             //DeadBand_1us; //dead timered:3us
EPwm1Regs.DBFED = DeadBand_1P5us;            //DeadBand_1us; //dead timefed:3us
EPwm1Regs.DBCTL.bit.OUT_MODE = 0x3;               // Deadband fully enabled

// SOCA used for sampling to convert ADC at EPWM_ET_ON_ZERO
EPwm1Regs.ETSEL.bit.SOCASEL = EPWM_ET_ON_ZERO;
// Generates SOCA pulse each event to have a 16kHz sampling frequency
EPwm1Regs.ETPS.bit.SOCAPRD = 1;

// Enables event
EPwm1Regs.ETSEL.bit.SOCAEN = 1;

//Clear Event Trigger flag
EPwm1Regs.ETCLR.bit.SOCA = 1;

//Enable TZ2 as one shot trip sources
EPwm1Regs.TZSEL.bit.OSHT2 = 1;

//Low level
EPwm1Regs.TZCTL.bit.TZA = TZ_FORCE_LO;
EPwm1Regs.TZCTL.bit.TZB = TZ_FORCE_LO;

EPwm1Regs.TZEINT.bit.OST = 0;
EPwm1Regs.TZCLR.bit.INT = 1;

// Force IGBT opened initial
EPwm1Regs.TZFRC.bit.OST = 1;
}