What am I missing here?

Projects Programming
1

Hello,

I am trying to generate a 30 KHz pulse using the following code...

/* Code to pulse pin 3 with a modulated signal
* Can be used to drive an IR LED to keep a TSOP IR reciever happy
* This allows you to use a modulated reciever and a continious beam detector
* By Mike Cook Nov 2011 - Released under the Open Source licence
*/

volatile byte pulse = 0;

ISR(TIMER2_COMPB_vect){                            // Interrupt Service Routine to pulse the modulated pin 3
    pulse++;
  if(pulse >= 8) {                                 // change number for number of modulation cycles in a pulse
    pulse = 0;                                     // Re-set Pulse value to zero
    TCCR2A ^= _BV(COM2B1);                         // toggle pin 3 enable, turning the pin on and off. TCCR2A register ^= Bitwise-shift (Compare Output Mode B, (Bit 5)) 
  }
}

void setIrModOutput(){                             // sets pin 3 going at the IR modulation rate
  //pinMode(3, OUTPUT);                              // Just enable output on Pin 3 and disable it on Pin 11
  TCCR2A = _BV(COM2A1) | _BV(WGM21) | _BV(WGM20);  
  TCCR2B = _BV(WGM22) | _BV(CS22);
  OCR2A = 66;                                      // defines the frequency 51 = 38.4 KHz, 54 = 36.2 KHz, 58 = 34 KHz, 62 = 32 KHz, 66 = 30KHz
  OCR2B = 33;                                      // deines the duty cycle - Half the OCR2A value for 50%
  TCCR2B = TCCR2B & 0b00111000 | 0x2;              // select a prescale value of 8:1 of the system clock
}

void setup(){
  setIrModOutput();
  TIMSK2 = _BV(OCIE2B);                            // Output Compare Match B Interrupt Enable
}

void loop()
{
pinMode(03, OUTPUT);                                // Just enable output on Pin 3 and disable it on Pin 11
}

But when I check the frequency with a scope I get 15000 Hz (see attached photo).

By my count there are three complete cycles ever .2 ms and this works out to 15 KHz. Can anyone explain what I am missing?

-Zeus

30 KHz - 07.04.14.JPG
30 KHz - 07.04.14.JPG1276×952 126 KB

2

It may just be me but it looks like the pinmode() command is in the wrong place...
L

3

pinMode(03, OUTPUT);OT:
It isn't your problem here, but it is fairly unusual and potentially risky to specify pin numbers in octal.

4

By using TCCR2A ^= _BV(COM2B1) you toggle the output each interrupt which which generates a square wave at half the interrupt frequency.

5

Thanks Guys,

I believe that I am going to have to end up moving the pinMode statement to the TImer ISR, but for now I just wanted to get my LED pulsing at the correct frequency so that I can start to build the detection loop that will run in the background.

I don't see how the pinMode placement could be cutting my frequency in half?

-Z

6

Thanks Stowite,

So what is the fix, do I compensate by adjusting my OCR2A & OCR2B values to account for this, or is this bad form?

  • Z
7

In order to generate a 30KHz square wave, you need to toggle the output state at 60KHz.

8

Why not just use the existing IR libraries IRLib or IRremote.

Altenatively, use the TIMER1 (TIMERONE) library which makes PWM easy.....unless you need to use Timer2.

9

This Thread may be of interest.

...R