Avr atmega16 fast pwm
This is a sample code to demonstrate how to generate a fast pwm wave with atmel atmega16 micro controller and the AVR LibC distributed in Debian 6.0 aka squeeze.
A basic knowledge of the counter in atmel cpu is required, for more info see the datasheet downloadable from atmel web site.
Let’s begin with the problem: I have a 4 Mhz micro controller and I want to create a 500 hz wave with variable PWM duty cycle. I use the 16 bit counter_1 because It is the only one I can set the frequency and the PWM duty cycle. The other 2 counters can be used in fast pwm too, but the frequency have to be related to the clock speed and the prescaler factor, not the one I freely choose.
The steps I follow are:
-
Prepare the port with counter pins out
-
Prepare the prescaler and choose the TOP value to obtain a 500hz wave. With an 4Mhz clock prescaled by 8 I have:
nstep = (4000000hz / 8) / 500hz = 1000
The counter start from BOTTOM (0) and ends after 1000 steps (TOP), then generate and interrupt (overflow) in which I change the duty cycle level, and restart again from BOTTOM.
-
Set the level of the PWM toggle between BOTTOM and TOP, I’ve used from 5% (50) to 95% (950).
-
enable the interrupt routine, start the counter and then do an infinite loop.
/* Copyright (C) 2010 Enrico Rossi
*
* This program is free software: you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation, either version 3 of the License, or
* (at your option) any later version.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with this program. If not, see <http://www.gnu.org/licenses/>.
*/
#include <stdint.h>
#include <stdlib.h>
#include <avr/interrupt.h>
#include <avr/io.h>
/* from 50 to 950 */
uint16_t duty_cycle;
/* IRQ routine at 500hz this is called 500 times x second */
ISR(TIMER1_OVF_vect)
{
/* if the duty is at 95% the restart from 5% */
if (duty_cycle > 950)
duty_cycle = 50;
/* increment the duty cycle by 1 step at the time wich is
0.1%, in this way I will complete 100% in 2 seconds (1000 step) */
duty_cycle++;
/* set the level where the output pin OC1A will toggle */
OCR1A = duty_cycle;
/* set the level where the output pin OC1B will toggle
this will be the inverse of OC1A level */
OCR1B = 1000 - duty_cycle;
}
void counter_setup(void)
{
/* Pin OC1A & B set to output */
DDRD |= _BV(PD4) | _BV(PD5);
/* Clear OC1A & B on compare match, set it at BOTTOM.
Waveform generation mode on 14, see datasheet */
TCCR1A = _BV(COM1A1) | _BV(COM1B1) | _BV(WGM11);
TCCR1B = _BV(WGM13) | _BV(WGM12);
/* TOP value*/
ICR1 = 1000;
/* start with a duty of 50% */
OCR1A = 500;
OCR1B = 500;
/* enable interrupt on timer overflow */
TIMSK = _BV(TOIE1);
}
int main(void)
{
/* prepare everything */
counter_setup();
/* start with a duty cycle of 50% */
duty_cycle = 500;
/* enable interrupt */
sei();
/* start the counter with prescaler = 8 */
TCCR1B |= _BV(CS11);
/* Infinite loop doing nothing, everything is
handled by the IRQ routine called 500 times x second. */
for (;;);
/* just for correct looking code, disable irq and terminate */
cli();
return(0);
}
compile with:
avr-gcc -I/usr/avr/include/ -Wall -Wstrict-prototypes -pedantic -mmcu=atmega16 -O2 -D F_CPU=4000000UL -o pwm.elf main.c
avr-objcopy -j .text -j .data -O ihex pwm.elf pwm.hex
and if you have a stk500 connected to the ttyUSB0 for example, write the code with
avrdude -c stk500v1 -p atmega16 -P /dev/ttyUSB0 -e -U flash:w:pwm.hex
Attaching an stk500 development board’s led 0 and 1 to the OC1A and OC1B pin, the two led should alternatively fade on and off. Or in alternatives, with a scope like DSO Nano the waveform can be watched changing the pwm’s duty cycle.