void setup()
 {
 int i;
 float t,u;
 for(t=0.0,i=0; i < NPWM; i++,t +=   3.14159/ (float) (NPWM-1))
   {  
   u = 65535.0 * sin(t);   // 1/2 wave sine lookup table, scaled to (16 bits - 1)
   l[i] =  (int)u;  
   }
...

//
// 20Khz SPWM, code has only 50uS to run, takes about 10uS
//
ISR(TIMER1_OVF_vect)        
{  
 long c;
 if (pcount == 0)
     if (v1low == 1)           // if first 1/2 wave..
     {
      TCCR1A =  _BV(COM1B1) | _BV(WGM11); // config output compare to suit
     }                        
   else
     {
      TCCR1A =  _BV(COM1A1) | _BV(WGM11);   // config O.C. to suit
     }
 c = (l[pcount] * vpwr) >> 16;   // scale sine wave by vpwr, 32 bit integer calcs. 16 bit shift fastest of all
 if (v1low == 1)                 // alternate between 2 output compare pins. Get a full 50Hz waveform
   {
     OCR1B = c;                    //  was c 1/2 out this pin
   }
 else
   {
     OCR1A = c;                    // and then 1/2 out this pin.
   }
 pcount ++;
 if(pcount >= NPWM)          // pcount  will = NPWM, at each 1/2 wave, at the start
   {
   pcount = 0;               // reset counter
   uf=1;                     // enable one PID control loop execution, in loop()
   if (v1low == 1)           // if first 1/2 wave..
     {
      v1low = 0;             // toggle to 2nd half wave
      sbi(PORTD,7);          //DSO pulse
     }                        
   else
     {
      v1low = 1;             // toggle to 1st half wave
      cbi(PORTD,7);
     }
   }