PWM(Pulse Width Modulation) in AVR Microcontroller (Atmega-8)

Aim:

Pulse Width Modulation (PWM) is a comparatively recent power switching technique for providing intermediate amounts of electrical power between fully on and fully off levels. Usually, digital pulses have same on and off time period, but in some situations we need the digital pulse to have more/less on time/offtime. In PWM technique, we create digital pulses with unequal amount of on and off state to get required intermediate voltage values.

Description:

 In my previous post, we have discussed the basic concepts of PWM. Let’s summarize it first:

• PWM stands for Pulse Width Modulation.
• It can be generated by comparing predetermined waveform with a reference voltage level or by making simple analog circuits.
• Duty Cycle of a PWM waveform is given by the following relation.

• There are three modes of PWM operation – Fast PWM, Phase Correct PWM and Frequency and Phase Correct PWM
• How to choose timer, operation mode and compare output mode for generating the desired PWM.

When you take a square wave, its on for a while and off for a while.  If we divide the on by the off and multiply by 100% we will get what is called a duty cycle.

Duty_Cycle = [ON_time / (ON_time + OFF_time) ] * 100

So if we are on for 1ms and then off for 1ms we will end up with a 50% duty cycle;  If we are on for 1ms and off for 3ms we end up with a 25% duty cycle.

Output_Voltage = Duty_Cycle * Input_Voltage

Now if we take our duty cycle and multiply it by our voltage we will get the output voltage.  So if we have a 5V power supply and we activate a PWM on a 25% duty cycle we will make an analog device behave as if it was receiving a 1.25V signal. Cool eh!?

Much like the counter functions PWM can be simulated in software however, the hardware version is preferred because it just sort of does its own thing and, with very little coding you can get a constant square wave going.

Remember the Prescaler? well its back in the PWM.  And much like in the counter, its roll is to slow things down.  This is good because it allows us to run the PWM at different frequencies.  This is important because some devices are sensitive to PWM speeds.  A motor for example will get hot if the PWM waveform is too fast, and will jitter if the PWM is too slow.  Since I already planted the question in your head, the answer is start at 10kHz.  Different motors like different frequencies but 10kHz will get you into the ball park.

The ATmega8 has 3 PWM outputs, 2 are located on timer/counter1 (16bit) and 1 is located on timer/counter2 (8bit).

As always, the output pin has the same limitations as any output (see the Digital Output Chapter for details).

This mode is recommended for motor control.

The frequency of the Phase Corrected PWM can be calculated by the following equation.

PWM_frequency = clock_speed / ( Prescaller_value * 510 )

	7 bit	6 bit	5 bit	4 bit	3 bit	2 bit	1 bit	0 bit
TCCR2	FOC2	WGM21	COM21	COM20	WGM20	CS22	CS21	CS20

Timer/Counter Control Register 2

CS22	CS21	CS20	DESCRIPTION
0	0	0	Timer/Counter2 Disabled
0	0	1	No Prescaling
0	1	0	Clock / 8
0	1	1	Clock / 32
1	0	0	Clock / 64
1	0	1	Clock / 128
1	1	0	Clock / 256
1	1	1	Clock / 1024

CS bits

MODE	WGM21	WGM20	DESCRIPTION
0	0	0	Normal
1	0	1	PWM Phase Corrected
2	1	0	CTC
3	1	1	Fast PWM

Waveform Generator Mode bits

COM21	COM20	DESCRIPTION
0	0	OC2 disabled
0	1	Reserved
1	0	None-inverted mode (HIGH at bottom, LOW on Match)
1	1	Inverted mode (LOW at bottom, HIGH on Match)

Applies only to PWM modes

	7 bit	6 bit	5 bit	4 bit	3 bit	2 bit	1 bit	0 bit
TIMSK	OCIE2	TOIE2	TICIE1	OCIE1A	OCIE1B	TOIE1	–	TOIE0

Timer/Counter Interrupt Mask Register

	7 bit	6 bit	5 bit	4 bit	3 bit	2 bit	1 bit	0 bit
TIFR	OCF2	TOV2	ICF1	OCF1A	OCF1B	TOV1	–	TOV0

Timer/Counter Interrupt Flag Register

	7 bit	6 bit	5 bit	4 bit	3 bit	2 bit	1 bit	0 bit
TCNT2

Timer/Counter Register (stores the counter value)

	7 bit	6 bit	5 bit	4 bit	3 bit	2 bit	1 bit	0 bit
OCR2

Output Compare Register (stores the compare value)

Block Diagram

Schematic

Code

// ***********************************************
// Project: Interfacing PWM to atmega8
// Author: Hack Projects India
// Module description: Operate array of LED's
// ***********************************************

#define F_CPU 8000000UL
#include <avr/io.h>
#include <util/delay.h>

void pwminit(void)
{
 TCNT1=0x00;//Initial counter value is 0
 TCCR1B = 0x01;//Timer with no prescalling
 TCCR1A = (1<<COM1A0) | (1<<COM1A1) |(1<<COM1B0) | (1<<COM1B1) | (1<<WGM10) | (0<<WGM11);// Timerr PWM mode
}

int main(void)
{
 int count=0;
 DDRC=0x00;
 DDRB=0xff;
 pwminit();
 OCR1A=255;
 OCR1B=255;
 while(1)
 {
  _delay_ms(50);
  OCR1A=count;
  count++;
  if(count>255)
  count=0;
 }
}

Downloads:

The code was compiled in Atmel Studio 6 and simulation was made in Proteus v7.7.
To download code and proteus simulation click here.

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