Is Timer1 Really Necessary For Motor Control On A Zumo?

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1

Based on the code for the Zumo library it would seem that using Timer1 for motor control is completely optional, or did I miss something here?

...
#ifdef USE_20KHZ_PWM
  OCR1B = speed;
#else
  analogWrite(PWM_L, speed * 51 / 80); // default to using analogWrite, mapping 400 to 255
#endif
...

If you can simply send a 0-255 PWM signal to the motor driver, then what is the advantage (i.e. the point) of using Timer1 in the first place?

ZumoMotors.cpp

#include "ZumoMotors.h"

#define PWM_L 10
#define PWM_R 9
#define DIR_L 8
#define DIR_R 7

#if defined(__AVR_ATmega168__) || defined(__AVR_ATmega328P__) || defined (__AVR_ATmega32U4__)
  #define USE_20KHZ_PWM
#endif

static boolean flipLeft = false;
static boolean flipRight = false;

// constructor (doesn't do anything)
ZumoMotors::ZumoMotors()
{
}

// initialize timer1 to generate the proper PWM outputs to the motor drivers
void ZumoMotors::init2()
{
  pinMode(PWM_L, OUTPUT);
  pinMode(PWM_R, OUTPUT);
  pinMode(DIR_L, OUTPUT);
  pinMode(DIR_R, OUTPUT);

#ifdef USE_20KHZ_PWM
  // Timer 1 configuration
  // prescaler: clockI/O / 1
  // outputs enabled
  // phase-correct PWM
  // top of 400
  //
  // PWM frequency calculation
  // 16MHz / 1 (prescaler) / 2 (phase-correct) / 400 (top) = 20kHz
  TCCR1A = 0b10100000;
  TCCR1B = 0b00010001;
  ICR1 = 400;
#endif
}

// enable/disable flipping of left motor
void ZumoMotors::flipLeftMotor(boolean flip)
{
  flipLeft = flip;
}

// enable/disable flipping of right motor
void ZumoMotors::flipRightMotor(boolean flip)
{
  flipRight = flip;
}

// set speed for left motor; speed is a number between -400 and 400
void ZumoMotors::setLeftSpeed(int speed)
{
  init(); // initialize if necessary
    
  boolean reverse = 0;
  
  if (speed < 0)
  {
    speed = -speed; // make speed a positive quantity
    reverse = 1;    // preserve the direction
  }
  if (speed > 400)  // Max 
    speed = 400;
    
#ifdef USE_20KHZ_PWM
  OCR1B = speed;
#else
  analogWrite(PWM_L, speed * 51 / 80); // default to using analogWrite, mapping 400 to 255
#endif 

  if (reverse ^ flipLeft) // flip if speed was negative or flipLeft setting is active, but not both
    digitalWrite(DIR_L, HIGH);
  else
    digitalWrite(DIR_L, LOW);
}

// set speed for right motor; speed is a number between -400 and 400
void ZumoMotors::setRightSpeed(int speed)
{
  init(); // initialize if necessary
    
  boolean reverse = 0;
  
  if (speed < 0)
  {
    speed = -speed;  // Make speed a positive quantity
    reverse = 1;  // Preserve the direction
  }
  if (speed > 400)  // Max PWM dutycycle
    speed = 400;
    
#ifdef USE_20KHZ_PWM
  OCR1A = speed;
#else
  analogWrite(PWM_R, speed * 51 / 80); // default to using analogWrite, mapping 400 to 255
#endif

  if (reverse ^ flipRight) // flip if speed was negative or flipRight setting is active, but not both
    digitalWrite(DIR_R, HIGH);
  else
    digitalWrite(DIR_R, LOW);
}

// set speed for both motors
void ZumoMotors::setSpeeds(int leftSpeed, int rightSpeed)
{
  setLeftSpeed(leftSpeed);
  setRightSpeed(rightSpeed);
}

.

2

Hello.

The motor control functions for the Zumo use hardware PWM outputs from Timer1 to generate pulse width modulation at a 20 kHz frequency. Generating a PWM output using analogWrite() on the Arduino Uno will also use a timer.

- Jeremy

3

Why does the library use code to setup the Timer1 registries for a 20kHz frequency instead of just using an analogWrite( at 500Hz)?
Does using a 20kHz frequency have some advantage over a standard PWM signal of 500Hz ?
If so then what is the advantage?

I am going to guess higher torque or maybe noise because 20KHz is on the outside of human hearing.
If so then why not set the frequency to 40kHz?

.

4

Setting the PWM frequency to 20kHz eliminates the whining sound heard when PWMing at lower frequencies, like the Arduino’s standard PWM frequency of 500Hz. There is power loss proportional to the switching frequency, which makes it less desirable to go much beyond 20kHz.

- Jeremy

5

Thanks!