unsigned long nlines = 0;

// length of message to generate
#define MLEN 60
char diagmsg[MLEN+1];

volatile unsigned long cnt = 0; // number of ISR routine calls so far 

void setup() {
  Serial.begin(500000);
  delay(100);

// generate timer 4 interrupt every 0.5ms 
 cli();
  TCCR4A = 0;
  TCCR4B = (1 << WGM42) | (1 << CS40) | (1 << CS42); // prescale by 1024 and set CTC mode 
  TCNT4  = 0;
  OCR4A = 8;  // compare with 8 to get interrupt every 0.5 ms
  TIMSK4 = (1 << OCIE4A);  // enable timer overflow interrupt (and no others)
  sei(); 
}

void loop() {
  nlines++;  
  char c = '0' + nlines % 10;
  int i=0;
  while(i<MLEN) {
    diagmsg[i] = c;
    i++;
  }
  diagmsg[i] = '\0';
  Serial.print(nlines);
  Serial.print(" ");
  Serial.print(diagmsg);
  Serial.print(" ");
  Serial.println(cnt);
}

// NLOOP controls duration of interrupt routine call
// this may need adjustment to demonstrate the issue
#define NLOOP 360
void test_isr() {
//  unsigned long start = micros();
  for (int i=0; i<NLOOP; i++) {
      cnt++;    
  }
}

ISR(TIMER4_COMPA_vect) {
  test_isr();
}