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ATMega328P generating PWM with timer2

Thread starter #1
Hello,

I'm trying to generate a PWM singlal with timer2 module of ATMega328P. Say I want to generate a 50% duty cycle signal. I have attached a DC motor at the output OOC2A(RB3) to see the value of the signal.

I wrote this code: Initialised TMR2 in fast pwm non inverting mode. My freq is 1Mhz and I used /8 prescaler option.
From my understanding OCR2A is the duty cycle when TMR2 is in PWM mode. Now since TMR2 is an 8-bit timer from my understanding writing 255 in OCR2A means 100% duty cycle. And 127 means ~50% duty cycle. So I wrote 127 in the OCR2A and expected a 50% duty cycle. Please correct me if I'm wrong here.
WHen I run the code the motor runs full speed no matter the value which I put in the OCR2A register :(. Am I missing something here?

Code:
/*
 * AVRGCC1.c
 *
 * Created: 2/19/2017 4:28:32 PM
 *  Author: Jarvis
 */ 

#include <avr/io.h>
#include <util/delay.h>
#define F_CPU 1000000L

void pwm_init()
{
    // initialize TCCR2 as: fast pwm mode, non inverting
    TCCR2A |= (1<<COM2A1) | (1<<COM2A0) | (1<<WGM21) | (1<<WGM20);
    TCCR2B |= (1<<CS21); // clkT2S/8 prescale
    // OC2RA pin made as output 
    DDRB |= (1<<PB3);
   
}


int main(void)
{
   
    pwm_init();
   
    while(1)
    {
     OCR2A = 127;    
    }
}
 

wkrug

Active Member
#2
The code seem's to be right.
Have You disconnected the motor and measured the signal with an oscilloscope?
You can simulate the code with AVR Studio 7.

You have attached a free wheeling diode to the Motor?
 
Thread starter #4
I've edited some stuff in the code and it runs fine now for small duty cycles. If I give it a high duty cycle the motor does not start.
 

wkrug

Active Member
#5
I think the PWM generation will not be the problem.
In the L293 datasheet i've seen that the t/on time ist about 700ns for the half amplitude of output signal.
I think that ist the time from input signal to output signal.
For the enable pin is explicid not time given in the datasheet.

I would suggest You to test the time from rising the enable pin to the output signal with an 2 channel oscilloscope.
Possibly at one point the pulses from the PWM are to short for the L293, and so the Motor wouldn't start.
 

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