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.

Answer 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.

• Areso: ` #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: 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