Electrical – PWM generator with adjustable duty cycle

One way to vary the duty cycle would be to remove R1 and connect a potentiometer track between V- and V+ with the wiper connected to R2/non-inverting input of the comparator. With the pot at mid position you will get 50% duty cycle but this will only be very roughly linear for small changes in position of the pot. Also, the frequency will reduce as you move the pot either side of mid-position.

Note that if you want single supply (5V) operation with the original circuit as shown, R1 would have to be returned to 2.5V rather than 0V.

Answered by joeymorin on AVRfreaks:

Note that on the Uno, the Arduino init code that runs before your setup() configures a lot of stuff, including all three timers on the 328P.

From wiring.c:

   sbi(TCCR0A, WGM01); 
   sbi(TCCR0A, WGM00); 
   sbi(TCCR0B, CS01); 
   sbi(TCCR0B, CS00); 
   sbi(TIMSK0, TOIE0); 

   sbi(TCCR1B, CS11); 
   sbi(TCCR1B, CS10); 
   sbi(TCCR1A, WGM10); 

   sbi(TCCR2B, CS22); 
   sbi(TCCR2A, WGM20);

This starts all three timers with a prescaler of 64.

TIMER0 is placed into mode 3 (fast PWM) with the overflow interrupt enabled to support the timing functions (millis(), micros(), and delay()).

TIMER1 is placed into mode 1 (fixed 8-bit phase-correct PWM).

TIMER2 is placed into mode 1 (phase-correct PWM).

void setup() 
{ 
  DDRB = (1<<PB1); // set pin 9 as output 

  TCCR1A |= (1<<COM1A1); 
  OCR1A = 125; 
} 

void loop() 
{ 
} 

Since WGM10 in TCCR1A is already set, setting COM1A1 will enable the PWM output in non-inverting mode, just as analogWrite() would.

TIMER1 is a 16bit timer, so it should overflow at 65536 ticks. From what I understand setting OCR1A between 0 and 65535 will change the duty cycle of the pulse. So, having set the OCR1A at 125, shouldn't I be getting an output of around 0.01 V instead of 2.5V? The results seem to imply that the clock is overflowing at 255.

In mode 1 it behaves like an 8-bit timer.

void setup() 
{ 
  DDRB = (1<<PB1); 

  TCCR1A |= (1<<COM1A1) | (1<<WGM11); 
  TCCR1B |= (1<<WGM13) | (1<<WGM12) | (1<<CS10); 

  ICR1 = 19999; 
  OCR1A = 10000; 
} 

void loop() 
{ 
}

Since WGM10 in TCCR1A is already set, setting WGM11, WGM13, and WGM12 will select mode 15, not mode 14. Mode 15 is fast PWM with TOP = OCR1A (not ICR1). Since you are also using setting OC1A output for PWM with COM1A1, this will result in an OC1A remaining high.

As mentioned already, if you want to configure timers in the Arduino environment, you should do it from scratch with = instead of |=.

theusch wrote:

And, for all I know, more might be run after setup()

From Arduino's main.cpp: Code:

#include <Arduino.h> 

int main(void) 
{ 
   init(); 

#if defined(USBCON) 
   USB.attach(); 
#endif 

   setup(); 

   for (;;) { 
      loop(); 
      if (serialEventRun) serialEventRun(); 
   } 

   return 0; 
}

JJ