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[on:]

Hi, new here and probably being a bit dim but I've Googled and done all the obvious stuff and I cannot figure out how to program the PWM to get a 40kHz signal to drive an ultrasonic sender for a distance measurement device.

I've got the example below running an giving me 8kHz but how do I scale this to get 40kHz. Also on my ATMega1280 the output is on pin 13, not pin 6 which this example code suggests (I guess the code is for a different processor). I've read the datasheet but it just went in and fell straight out again. Any help appreciated.

Code:

#include <avr/io.h>

int main(void)
{
    DDRD |= (1 << DDD6);
    // PD6 is now an output

    OCR0A = 128;
    // set PWM for 50% duty cycle


    TCCR0A |= (1 << COM0A1);
    // set none-inverting mode

    TCCR0A |= (1 << WGM01) | (1 << WGM00);
    // set fast PWM Mode

    TCCR0B |= (1 << CS01);
    // set prescaler to 8 and starts PWM


    while (1);
    {
        // we have a working Fast PWM
    }
}

[Reply #1 on:]

You need a pwm mode that allows you to set the top.

[Reply #2 on:]

You need a pwm mode that allows you to set the top.

Was the any more to your post, dhenry?

[Reply #3 on:]

You need to carefully read Nick Gammon's web site, I'd suggest starting here:

Nick Gammon's description of Timers and Counters

[Reply #4 on:]

PWM frequency on the Playground:
http://playground.arduino.cc//Code/PwmFrequency

Please keep in mind that changing the PWM frequency changes the Atmega's timers and disrupts the normal operation of many functions that rely on time (delay(), millis(), Servo library).

[Reply #5 on:]

You need a pwm mode that allows you to set the top.

Which mode is that?

[Reply #6 on:]

One of my sketches on the page below generates 50 KHz:

http://www.gammon.com.au/forum/?id=11504&reply=6#reply6

By changing this line you should be able to get 40 KHz:

Code:

const long frequency = 50000L;  // Hz

[Reply #7 on:]

Yep, it does, looks pretty accurate too:



Sketch:

Code:

const byte LED = 3;  // Timer 2 "B" output: OC2B

const long frequency = 40000L;  // Hz

void setup()
 {
  pinMode (LED, OUTPUT);

  TCCR2A = _BV (WGM20) | _BV (WGM21) | _BV (COM2B1); // fast PWM, clear OC2B on compare
  TCCR2B = _BV (WGM22) | _BV (CS21);         // fast PWM, prescaler of 8
  OCR2A =  ((F_CPU / 8) / frequency) - 1;    // zero relative 
  OCR2B = ((OCR2A + 1) / 2) - 1;             // 50% duty cycle
  }  // end of setup

void loop() { }

[Reply #8 on:]

Thanks for the replies, I'll give that a go

[Reply #9 on:]

Sorry to resurrect an old(ish) thread but what is the best way to stop and start this timer? I am using it for ultrasonic distance measurement so I need to send out a short ping and then listen for the echo. The ping needs to be short - is it best to just set the pin to input from output or do I need to stop the PWM, if so how?

[Reply #10 on:]

Turning the pin to input is probably the simple way to stop it.

The thing that actually starts the timer is the CS21 part of this line (the clock source):

Code:

  TCCR2B = _BV (WGM22) | _BV (CS21);         // fast PWM, prescaler of 8

So the above line starts it. This would stop it:

Code:

  TCCR2B = _BV (WGM22) ;         // fast PWM, timer off

[Reply #11 on:]

The full doc gives all the registers and commands, 55 pages of answers for those who ain'ts gots:

http://www.atmel.com/search.aspx?q=+ATMEGA328P&as_q=inmeta%3Aasset_type~Datasheets%2Binmeta%3Aproduct_family%3DAtmel%2520AVR%25208%252D%2520and%252032%252Dbit&dnavs=inmeta%3Aasset_type~Datasheets%2Binmeta%3Aproduct_family%3DAtmel%2520AVR%25208%252D%2520and%252032%252Dbit&filter=0&btnG=Search&client=support_frontend&proxystylesheet=support_frontend&output=xml_no_dtd&getfields=*&oe=UTF-8&ie=UTF-8&ud=1&num=1000&sort=date%3AD%3AL%3Ad1&lr=lang_en&exclude_apps=1&site=en_collection

BTW that's in thanks to your code in reply #7 Nick.

[Reply #12 on:]

Thanks for the replies

[Reply #13 on:]

Apologies for reviving this thread again, but I am now trying to use a similar bit of code to generate a 1kHz signal, however it is fine down to about 8kHz but it won't go below that. I suspect that it is due to the fast PWM but I have read the datasheet and I haven't got a clue how to stop using the Fast PWM.

Any help greatly appreciated.

[Reply #14 on:]

Why use PWM at all? Just set up a timer and interrupt to change the pin state.