PWM

PWM fundamental

1.                  Introduction: PWM stands for Pulse Width Modulation. In ATMEGA 32 we get this type of output from Pin numbers 4 and 21. It is important to control effective output voltage level. This type of output is helpful in producing variable luminance of LEDs, speed of dc motors etc.

2.                  Principle: It is a pulse train at equal time intervals of which on time duration is controlled. Effective voltage in PWM may be calculated by multiplying the pulse magnitude to the duty cycle (i.e. percentage ratio of on time to the total time period). For example 5 volt magnitude PWM with duty cycle 45% will produce the effective voltage 5X0.45 = 2.25 volts.

3.                  Modes: A reference voltage is compared with a sawtooth wave. Pulses produced with the difference between them are divided into three modes of operation. It is inverted mode if pulse has trailing edge as the waveform exceeds the reference voltage. The opposite is the non-inverted mode. And under toggle modes of operation the pulse toggles between two levels once the waveform exceeds the reference level.

4.                  BOTTOM - TOP – MAX: Realization using timer sets the initial value of the timer say BOTTOM. Then timer starts counting to the maximum count say MAX. for 8 bit timers TIMER0 and TIMER 2 MAX=255 and for TIMER1 MAX=65536. Under CTC mode timer may be cleared by compare its value below the MAX called the TOP.

5.                  PWM modes with Timers: Three modes of operations. Fast PWM, Phase Correct PWM, Frequency and Phase Correct PWM. In Fast PWM the timer counts from the BOTTOM to the TOP. In between the pulse starts when the sawtooth wave crosses beyond the reference level. Pulse ends with timer value match to the TOP. In Phase Correct PWM instead of sawtooth reference voltage is compared to a triangular wave. Thus PWM pulse produced here has equal interval of the center of the pulse i.e. phase of the pulse is corrected whereas in the last case pulse ends at equal interval with unequal distance between center of the pulse due change in reference voltage and it took less time i.e. faster PWM. In Frequency and Phase Correct PWM the sawtooth waveform has variable TOP (<=MAX) thus it basically represents a Fast PWM with variable frequency.

6.                   

Expt-1

1.      Title: Generate a 50 Hz PWM signal having 45% duty cycle.

2.      Components required:

Sl. No.	Category	Item name/value	Quantity	Circuit diagram
1	IC	ATMEGA32	1
2		7805	1
3	Resistor	10K	1
4		330R	1
5	Capacitor	0.1uf	2
6		22pf	2
7		10uf, 24Volt	1
8	Crystal Oscillator	16MHz	1
9	LED	Green	1
10	Battery	9Volt	1

3.      Procedure to code:

a.       Assign duty = 115; as duty cycle = 45% of 255 = 114.75 = 115

b.       Initialize timer in PWM mode

                                                                           i.      Initialize TCCR0 as per requirement, say as follows

                                                                          ii.      Assign PB3 as output port to make OC0 pin (pin PB3 for atmega32) as output pin

c.        Run forever in finite loop

                                                                           i.      Assign OCR0 by duty

4.      Code:

#ifndef F_CPU

#define F_CPU 16000000L

#endif

#include <avr/io.h>

#include <util/delay.h>

void pwm_init()

{

TCCR0 |= (1<<WGM00)|(1<<COM01)|(1<<WGM01)|(1<<CS00);

DDRB |= (1<<PB3);

}

void main()

{

uint8_t duty;

duty = 115;

pwm_init();

while(1)

{

OCR0 = duty;

}

}

5.      Circuit:

6.      PWM Simulation:

https://youtu.be/MyoptL0rQro

Expt-2

1.      Title: Program to change brightness of an LED.

2.      Components required:

Sl. No.	Category	Item name/value	Quantity	Circuit diagram
1	IC	ATMEGA32	1
2		7805	1
3	Resistor	10K	1
4		330R	1
5	Capacitor	0.1uf	2
6		22pf	2
7		10uf, 24Volt	1
8	Crystal Oscillator	16MHz	1
9	LED	Green	1
10	Battery	9Volt	1

3.      Procedure to code:

a.       initialize timer0 in PWM mode

                                                                           i.      initialize timer0 in PWM mode

                                                                          ii.      make sure to make OC0 pin (pin PB3 for atmega32) as output pin

b.       run forever

                                                                           i.      increasing brightness

                                                                          ii.      set the brightness as duty cycle

                                                                        iii.      delay so as to make the user "see" the change in brightness

                                                                        iv.      decreasing brightness

                                                                         v.      set the brightness as duty cycle

                                                                        vi.      delay so as to make the user "see" the change in brightness

                                                                      vii.      repeat this forever

4.      Code:

#ifndef F_CPU

#define F_CPU 16000000L

#endif

#include <avr/io.h>

#include <util/delay.h>

void pwm_init()

{

TCCR0 |= (1<<WGM00)|(1<<COM01)|(1<<WGM01)|(1<<CS00);

DDRB |= (1<<PB3);

}

void main()

{

uint8_t brightness;

pwm_init();

while(1)

{

for (brightness = 0; brightness < 255; ++brightness)

{

OCR0 = brightness;

_delay_ms(10);

}

for (brightness = 255; brightness > 0; --brightness)

{

OCR0 = brightness;

_delay_ms(10);

}

}

}

5.      Circuit:

6.      Simulation:

https://youtu.be/gE1W3G4fOtw