Problem with PWM interference

Question

I have a following code

  #include <Servo.h> //library for servo drive
  int MotorAPinA = 9;   //pin number for DC drive
  int MotorAPinB = 10;  //pin number for DC drive
  Servo myservo; //create object myservo, class servo from Servo library

void setup() {
  // put your setup code here, to run once:
  pinMode(MotorAPinA, OUTPUT);
  pinMode(MotorAPinB, OUTPUT);
  //myservo.attach(6); //pin number for servo output
  myservo.write(65);
}

void loop() {
  analogWrite(MotorAPinA, 240);
}

This very code works only partly.
If I comment line

myservo.attach(6)

then I got my DC drive working.
If I uncomment this line, my Servo goings crazy (effect looks like myservo.write(0) constantly), until I comment line

analogWrite(MotorAPinA, 240);

and then I got my servo working properly. While myservo.attach(6) is uncommented, DC drive doesn't work (with uncommented analogWrite).

So, basically, I can have or PWM or Servo working simultaneously, but not both

Any ideas? Arduino Uno R3, Arduino 1.8.1.

KIIV · Accepted Answer · 2017-02-12 09:38:53Z

1

Well, your PWM outputs are both on 16b Timer/Counter 1 and the servo is attached to the 8b Timer/Counter 0. So my guess is that 8bit counter has too low resolution for driving the servo and it uses Timer/Counter 1 in full 16b mode for better precision instead (PWM uses just 8bits from 16). Also Timer/Counter 0 is used for counting millis, so its settings can't be changed anyway.

For the confirmation you can either:

use pins 5 and 6 (TC0) or/and pins 3 and 11 (TC2) and for the servo use pin 9 or 10
You can read the values of TC1 configuration registers and according to the datasheet you can figure out what mode is currently used.

EDIT: So on the Uno board is used Timer/Counter 1 for timing purposes:

  if(timer == _timer1) {
    TCCR1A = 0;             // normal counting mode
    TCCR1B = _BV(CS11);     // set prescaler of 8
    TCNT1 = 0;              // clear the timer count
    TIFR |= _BV(OCF1A);      // clear any pending interrupts;
    TIMSK |=  _BV(OCIE1A) ;  // enable the output compare interrupt
    timerAttach(TIMER1OUTCOMPAREA_INT, Timer1Service);
  }

So no PWM on the OC1A nor OC1B in normal mode and the channel A is taken by Servo.

edited Feb 12, 2017 at 9:38

answered Feb 12, 2017 at 9:28

KIIV

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` #include <Servo.h> //library for servo drive int MotorAPinA = 6; //pin number for DC drive int MotorAPinB = 5; //pin number for DC drive Servo myservo; //create object myservo, class servo from Servo library void setup() { // put your setup code here, to run once: pinMode(MotorAPinA, OUTPUT); pinMode(MotorAPinB, OUTPUT); myservo.attach(9); //pin number for servo output myservo.write(65); } void loop() { analogWrite(MotorAPinA, 240); //analogWrite(MotorAPinB, 0); } ` somehow `` doesn't code code

Areso
– Areso

2017-02-12 13:27:00 +00:00
Commented Feb 12, 2017 at 13:27
Now it works. Servo on Timer/Counter1 16 bit (10 and 9 pins), PWMs on Timer/Counter0 0 bit (6 and 5 pins), and there are still free 11 and 3 pins of Timer/Counter2

Areso
– Areso

2017-02-12 13:30:19 +00:00
Commented Feb 12, 2017 at 13:30

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