Using 0C2 interrupt generate a pulse width modulated signal

Using 0C2 interrupt, generate a pulse width modulated signal to obtain a desired average analog voltage. The frequency and the desired voltage will be given to you in the exam by the TA. For practice, try 100 Hz, 3.4 volts.

Solution

There are various terms associated with PWM:

Period

As shown in the the figure, Ton denotes the on-time and Toff denotes the off time of signal. Period is the sum of both on and off times and is calculated as shown in the equation below:

Duty Cycle

Duty cycle is calculated as on-time to the period of time. Using the period calculated above, duty cycle is calculated as:

Voltage regulation is done by averaging the PWM signal. Output voltage is represented by the following equation:

As you can see from the equation the output voltage can be directly varied by varying the Ton value.

If Ton is 0, Vout is also 0. if Ton is Ttotal then Vout is Vin or say maximum.

here, Vin =5V

Vout = 3.4 V

therefore, D=(3.4/5) = 0.68

D=0.68*255= 173D=AE hex

T_on= D*T_total= 6.8 ms

AVR: Assembly Code Example

;8-bit Non-Inverted PWM code example

.equ pulse_width = $0AE

;Pulse width can be changed from 0 to FF(255)

PWM_START:

ldi temp, pulse_width ;Load pulse width

out OCR1AL, temp ;OCR1A = Pulse width

clr temp

out OCR1AH, temp

ldi temp, $81 ;8-bit PWM Mode

out TCCR1A, temp ;Non Inverted

in temp, DDRD ;Make PortD.5 as o/p

ori temp, (1<<5)

out DDRD, temp

ldi temp, $1 ;Start PWM

out TCCR1B, temp

ret ;Return to main

;PWM will run in background automatically