pwm同步输出选择

在PWM_PSFB_VMC_SR_Cnf.c中epwm1和epwm2的同步输出选择为什么不一样？这样做有什么考虑吗？

 (*ePWM[n]).TBCTL.bit.PRDLD = TB_IMMEDIATE;   // set Immediate load 
 (*ePWM[n]).TBPRD = period;
 (*ePWM[n]).CMPA.half.CMPA = period/2;    // Fix duty at 50% 
 (*ePWM[n]).TBPHS.half.TBPHS = 0;
 (*ePWM[n]).TBCTR = 0;

 (*ePWM[n]).TBCTL.bit.CTRMODE = TB_COUNT_UP;
 (*ePWM[n]).TBCTL.bit.PHSEN = TB_DISABLE;
 (*ePWM[n]).TBCTL.bit.SYNCOSEL = TB_CTR_ZERO;   //used to sync EPWM(n+1) "down-stream"
 (*ePWM[n]).TBCTL.bit.HSPCLKDIV = TB_DIV1;
 (*ePWM[n]).TBCTL.bit.CLKDIV = TB_DIV1;

 (*ePWM[n]).AQCTLA.bit.ZRO = AQ_SET;
 (*ePWM[n]).AQCTLA.bit.CAU = AQ_CLEAR;
 
 (*ePWM[n]).DBCTL.bit.OUT_MODE = DB_FULL_ENABLE;
 (*ePWM[n]).DBCTL.bit.POLSEL = DB_ACTV_HIC;   // Active Hi Complimentary

 (*ePWM[n]).DBRED = 20;        // dummy value for now
 (*ePWM[n]).DBFED = 20;        // dummy value for now

// ePWM(n+1) init.  EPWM(n+1) is a slave
 (*ePWM[n+1]).TBCTL.bit.PRDLD = TB_IMMEDIATE;  // set Immediate load
 (*ePWM[n+1]).TBPRD = (period-1);     
 (*ePWM[n+1]).CMPA.half.CMPA = period/2;    // Fix duty at 50%
 (*ePWM[n+1]).TBPHS.half.TBPHS = 0;      // zero phase initially
 (*ePWM[n+1]).TBCTR = 0;

 (*ePWM[n+1]).TBCTL.bit.CTRMODE = TB_COUNT_UP;
 (*ePWM[n+1]).TBCTL.bit.PHSEN = TB_ENABLE;
 (*ePWM[n+1]).TBCTL.bit.SYNCOSEL = TB_SYNC_IN;   // Sync "flow through" mode
 (*ePWM[n+1]).TBCTL.bit.HSPCLKDIV = TB_DIV1;
 (*ePWM[n+1]).TBCTL.bit.CLKDIV = TB_DIV1;