Using an AVR timer to generate a 38 kHz signal

avrcdelaymicrocontrollertimer

I need a 13 microseconds delay for transmitting IR at a 38 kHz carrier wave. My chip (ATtiny84) is running at 8 MHz. I cannot figure out what's the problem is in my code:

void send(long microsecs)
{
    TCCR0B |= (1 << CS00); // Enable Timer 0 - No prescale

    while (microsecs > 0)
    {
        // 38 kHz is about 13 microseconds high and 13 microseconds low
        PORTB |= (1 << PB2); // 3 µs
        TCNT0 = 0;
        while (TCNT0 < 104);

        PORTB &= ~(1 << PB2);   // 3 µs
        TCNT0 = 0;
        while (TCNT0 < 104);

        // So 26 microseconds altogether
        microsecs -= 26;
    }
}

The delay (104 clock cycles) sums up to 13 microseconds @8 MHz and the while loop should hang up the code for the required time period, but it's not working.

But

When I replace this:

   TCNT0 = 0;
   while (TCNT0 < 104);

by this:

_delay_us(13);

everything works fine.

If I'm doing something wrong, please point out.

Best Answer

You are not updating PB2, you set it high in the 1st delay and then you set it high again in the second delay, switch from PORTB |= (1 << PB2); to PORTB &= ~(1 << PB2); in one of the delays

Related Solutions

Electronic – AVR timer speedup on ATmega328

For a quick debug, I would try to do the same thing using Arduino Library for TLC5940 and see if it's getting fast or not. If it works with the library, you may check its source and compare with yours. Since you're familiar with AVR, you should easily convert Arduino source to native AVR.

Just in case you don't know how to upload compiled Arduino sketches to AVR: When you compile your sketch, it creates a hex file (you can see the exact location of the file by turning on verbose mode in settings). You can upload that hex to your AVR with your favorite programmer.

Hope it helps

Electronic – AVR – ATmega16 – Timer config

I am trying to create a TickCount.

You'll need a global variable(big enough) to hold the number of seconds(49.7*24*60*60*1000 = 32bit long value OR a ulong) elapsed, increment it inside your isr( it seems you did this part right !).

volatile uint32_t elapsed_ms;

then you'll need function that returns this value.

uint32_t TickCount(void){
 return elapsed_ms;
}

Now we come to part two:
TickCountSys increases once every milli second

If you want to do it with 8MHz Xtal + Timer 0 Overflow, then you cannot achieve precise 1ms ticks. Reason being 8000 clock pulses can not overflow the timer 0. If you use no prescalar then, T0 overflows = 8000/256 = 31.25 times. if you prescale the timer clock with T/8 setting then 1000/256 = 3.90625 close to 4 Overflows.

Assuming that you chose to go ahead with the T/8 pre-scalar, then you'll need to modify the isr body to something like

      static uint8_t temp_count=0;
        temp_count++;
      if(temp_count >= 4){
        temp_count = 0;
        elapsed_ms++;
        //add a method here to roll the 49.7 day completion.
      }

That's it.
From your code it seems you've done the timer and interrupt settings correctly. But your code is quite fuzzy and I failed to locate any function named TickCount or similar.

I'd suggest you to start off with a simple code, like blink a led on timer0 overflow( with a HIGH PRESCALAR value like T/1024). Then gradually add on the code logic. Good Luck.

Best Answer

Related Solutions

Electronic – AVR timer speedup on ATmega328

Electronic – AVR – ATmega16 – Timer config

Related Topic