How do i generate exact number of PWM pulses in arduino?

Question

So my code here generates a PWM wave of 38Khz on Port B pin 7 of arduino Mega 2560 . For that I have used a prescaler of 8 and then set the ICR1 register value to 51 . I have also set the WGM10 ,WGM11, WGM12 and WGM13 bits to 1 , to enable fast PWM mode with Top reference value as ICR1 contents . Similarly , COM1A1 is set , to enable clear output on compare equal else high . CS11 is set to high to enable the prescaler 8 .

My code is as follows -

#include<avr/io.h>
#include <avr/iomxx0_1.h>
#include<util/delay.h>
void pwm_init()
{
 // Configure PB.7 as output pin
 DDRB |= (1<<PB7);
 TCCR1A = (1<<WGM11) | (1<<WGM10) | (1<<COM1A1);
 TCCR1B =(1<<WGM12) | (1<<WGM13) | (1<<CS11);
 ICR1 = 51;
}
void pwm_generate()
{
 OCR1A = (50 * 52)/100 ;
}
int main()
{
pwm_init();
pwm_generate();
}

What I require from the program is that , it should stop producing pwm pulses once a sufficient number of pulses are transmitted . For example - The program should output 7 pwm pulses of 38Khz and then it should give logic 0 as output for specified time period . And then after that specified period , it should produce 38 Khz pwm wave again of say, 5 pulses and then again logic 0 for certain duration . And so on ... How do i achieve this ?

PS- This program is actually being written to simulate a pwm wave required by the IR transmitters in TV remote . The PWM pulses represent the ON period and the Logic 0 conditions would represent the gap in between two successive train of PWM pulses.

Picture of waveform for reference -

• Majenko: There's a whole IRremote library to do that for you.
• Boudhayan Dev: I don't want to use any libraries.
• Majenko: It's not the number of carrier cycles that matters but the time the pulses are on or off for _ and that not accurate to a pulse. The carrier is just to get the signal through a band-pass filter at the receiver end.

Answer 1

One way to get a fixed number of pulses is to set up an interrupt handler that is invoked once per toggle (or, once per cycle, whichever is appropriate). It can decrement a counter, and shut off an output pin when the counter gets to zero. Or could set the output pin low and the CS bits to 0 to stop the counter. For example, if timer 1 is in use:

volatile byte TogCounter;
...
// In setup():
TIMSK1 = 1<<TOIE1; // Enable timer 1 overflow interrupt at TOP
...
// Define a timer 1 ISR
ISR(TIMER1_OVF_vect) { // ISR for timer 1 overflows
 if (TogCounter) { // Don't decrease if not counting
 --TogCounter;
 if (!TogCounter) {
 pinMode (TogPin, INPUT); // Disable output when count runs out
 // If desired, set a volatile wavetrain-is-done flag here
 }
 }
}

To output a specified number of cycles n, set the mode of TogPin to OUTPUT and set TogCounter to 2·n (or to n if counting cycles instead of toggles). Note, waveforms will be of more-consistent length if you also clear the counter register just before starting any pulse train.

To generate a sequence like m₁ cycles on, n₁ off, m₂ cycles on, n₂ off, etc, you could set up an array containing m₁, n₁, m₂, n₂, etc; then each time TogCounter runs down, toggle TogPin from INPUT to OUTPUT or vice versa, and load the next array entry into TogCounter.

• Boudhayan Dev: Thank you. I have used the CS reg to set the port to normal and forced logic 0 through it after specific number of pulses were generated . Though I haven't used interrupt , your explanation was helpful .

Answer 2

simple:

1) generate a pulse train of 38Khz -> many ways to do that;

2) put a resistor on the pulse train -> something like 1K will do it;

3) on the other end of the resistor, put a GPIO pin on it; configure it as input, but write a '0' to it;

4) route the clock input of a timer to that GPIO pin; set it to trigger an isr on a given number of pulses -> many ways to do that;

5) in the isr set up in 4), turn the gpio pin in 3) to output to disable the pulse train; input to enable the pulse train.

6) done.

• Boudhayan Dev: Can you elaborate with the isr code example ?