TMS570LS0714: ADC Trigger source cannot change to Altemate Trigger????

Part Number: TMS570LS0714

dear all:

         I used  etpw(1 5 7)  product pwm to control motor.  ADC group sample UVW current.

but ADC Trigger source   cannot change to Altemate Trigger?

  External connection  pin（other etpwm ）is necessary to trigger adc？？？？？