Timer interrupt problem

Projects Programming
1

hi
I have a question.
Why can't I dirve the servo with this sketch:

///////////////////////////////////////////////

#include <avr/interrupt.h>
#include <Adafruit_PWMServoDriver.h>

Adafruit_PWMServoDriver pwm = Adafruit_PWMServoDriver();

#define MAX 419
volatile int servo=171;

void setup() {
Serial.begin(9600);
pwm.begin();
pwm.setPWMFreq(50);
cli();
TCCR3A = 0;
TCCR3B = 0;
TCNT3 = 0;
OCR3A = 50000;
TCCR3B |= (1<<CS31);
TCCR3B |= (1<<WGM32);
TIMSK3 |= (1<<OCIE3A);
sei();
}

ISR(TIMER3_COMPA_vect)
{
if(servo<=MAX)
{servo++;
pwm.setPWM(0, 0, servo);
}

else{
servo = 171;
}
}

void loop(){}

/////////////////////////////////////////

Used software: Arduino
I'm using an Arduino MEGA 2560.
For the servo, I use the adafruit-16-channel-pwm-slash-servo-shield.

Without using timer, the servo drives.

So what's wrong here??

Thanks in advance

2

Have a look here, especially under pitfalls, to see if your choice of pins is compatible with the timers you are using for the Mega:
https://arduino-info.wikispaces.com/Timers-Arduino