https://github.com/cloud-rocket/DuePWM

sketch_nov29a.ino

#include <DuePWM.h>


#define PWM_FREQ1  20
#define PWM_FREQ2  20


// DuePWM can be instantiated using a default constrcutor like this:
// DuePWM pwm();
// OR
// DuePWM pwm;
// In these cases it will use the default values of PWM_FREQUENCY
// defined in variant.h which is currently 1kHz and is equivalent to:
// DuePWM pwm(PWM_FREQUENCY, PWM_FREQUENCY);
DuePWM pwm( PWM_FREQ1, PWM_FREQ2 );

uint32_t pinl = 6;
uint32_t pinr = 7;

uint32_t current;
int val = 0;
int interruptPin = 2;

void setup() 
{
  Serial.begin(9600);
    pwm.setFreq1( PWM_FREQ1 );
    pwm.setFreq2( PWM_FREQ2 );
    pwm.pinFreq1( pinl );
    pwm.pinFreq2( pinr );
    //pwm.stop(pinr);
    delay(1000);
    current = pinl;
    attachInterrupt(digitalPinToInterrupt(interruptPin), onInterrupt, CHANGE);
}

void loop() 
{  
  int tmp = analogRead(A0)/4;
  if(tmp < 10)
    tmp = 0;
  if(tmp > 250)
    tmp = 250;
    if(tmp != val){
      val = tmp;
      pwm.pinDuty(current, val);
   }
  delay(100);
  Serial.print(current);
  Serial.print(" ");
  Serial.println(val);
}

void onInterrupt(){
  pwm.pinDuty(current, 0);
  current =  digitalRead(interruptPin) == HIGH ? pinl : pinr;
  
}