`servo.attach()` paralyzing (one) motor on L298N

Question

I have 2 DC motors (well, 4; but each pair is controlled by the same "signals") connected via a L298N, which is connected to the Arduino. In addition, I have a servo motor connected to the Arduino.

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When I run the following program (in reality, it's a bigger program; however, I've managed to chiseled it down to the following):

#include <Servo.h>

const int servoPin = 3;
Servo servo;

const int LMotorsSpeedPin = 5;
const int RMotorsSpeedPin = 10;
const int LMotorDirectionBit1Pin = 7;
const int LMotorDirectionBit2Pin = 6;
const int RMotorDirectionBit1Pin = 8;
const int RMotorDirectionBit2Pin = 9;
const int MotorSpeed = 150;

void setup()
{
  // 1
  pinMode(servoPin, OUTPUT);
  servo.attach(servoPin);

  // 2
  pinMode(LMotorsSpeedPin, OUTPUT);
  pinMode(RMotorsSpeedPin, OUTPUT);
  pinMode(LMotorDirectionBit1Pin, OUTPUT);
  pinMode(LMotorDirectionBit2Pin, OUTPUT);
  pinMode(RMotorDirectionBit1Pin, OUTPUT);
  pinMode(RMotorDirectionBit2Pin, OUTPUT);

  // 3
  digitalWrite(LMotorDirectionBit1Pin, LOW);
  digitalWrite(LMotorDirectionBit1Pin, HIGH);
  analogWrite(LMotorsSpeedPin, MotorSpeed);

  // 4
  digitalWrite(RMotorDirectionBit1Pin, LOW);
  digitalWrite(RMotorDirectionBit1Pin, HIGH);
  analogWrite(RMotorsSpeedPin, MotorSpeed);

  // 5
  delay(10000);
}

void loop() {}

• 1 Sets up servo.
• 2 Sets up L298N pins.
• 3 Sets up left motor data.
• 4 Sets up right motor data.
• 5 Tests the code above for 10 [s].

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I assume that you'll need some experience with the L298N; however, here's the gist.

*MotorsSpeedPin sends each motor what speed (MotorSpeed 0-255) they must go (via PWM). Data into a pair of *MotorDirectionBit*Pin tell each motor what direction to go (i.e. back 01, stop 00 11, right 10).

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When I run the program above, only the left motor runs. However, if I comment out 1 (which essentially renders the servo useless), both left and right motors start to work.

I find this problem quite odd. What does the servo code have to do with the motor code (and only one at that!)?

Answer 1

The Servo library uses Timer 1 to create the servo control signal. That means it can run the servo on any pin or combination of pins, and at the 50Hz speed that a servo likes best.

However, pins 9 and 10 also use Timer 1 to create the PWM signal.

It can't do both.

As soon as you start using the Servo library you lose PWM on pins 9 and 10.

So what can you do about it? Simply move your pins around to different pins. You can't use 9 and 10 for PWM, but you can use them for the servo. So put your servo on pin 10 and your right motor speed control on pin 3 and all should be happy.