Changing the PWM frequency to 20kHz on a Mega

Using Arduino Programming Questions
1

Hello there,

I'm powering a motor from my Arduino Mega through a Pololu driver. When I send the signal, the motor makes a high-pitch noise which is very annoying, probably due to a low frequency PWM.

The driver supports up to 20kHz, so I'd like to change the frequency to that value. My programming experience is limited, so any help/advice would be super appreciated.

Cheers,

Carlos

2

Which pins? There are three timers, one is used by millis(), micros() and delay().

The other ones you can speed up fairly easily by changing the prescaler, from memory.

3

I was using pins 8 & 9 on the Mega, which supply a PMW output. But I can obviously change them if they interfere with those functions (I'm using them).
Is there a way of knowing which timer control which pins?

How's the pre-scaler changed?

Thank you!

4

colabarris:
I was using pins 8 & 9 on the Mega, which supply a PMW output. But I can obviously change them if they interfere with those functions (I'm using them).
Is there a way of knowing which timer control which pins?

The Mega 2560 reference schematic, available from this site. In part:

1025×937 186 KB

You can see from that, that one is on timer 2 (OC2B) and one on timer 4 (OC4C).

How's the pre-scaler changed?

To simplify things, change to pins 9 and 10 which are both on timer 2 (OC2A and OC2B).

Then add a couple of lines of code to alter the prescaler:

void setup ()
  {
  TCCR2B &= ~ _BV (CS22); // cancel pre-scaler of 64
  TCCR2B |= _BV (CS21);   // use pre-scaler of 8
  analogWrite (9, 50);    // 19.6 % duty cycle
  analogWrite (10, 200);  // 78.4 % duty cycle
  }  // end of setup

void loop () { }

Results:

The predicted duty cycles show up, with a PWM frequency of 3.92 KHz.

5

Or you can use no pre-scaler, which gives a PWM frequency of 31.37 KHz:

void setup ()
  {
  TCCR2B &= ~ _BV (CS22); // cancel pre-scaler of 64
  TCCR2B |= _BV (CS20);   // no pre-scaler
  analogWrite (9, 50);    // 19.6 % duty cycle
  analogWrite (10, 200);  // 78.4 % duty cycle
  }  // end of setup

void loop () { }
6

Wow, thanks a lot, Nick. That was of great help and quite easy!

Just one last question: if I don't use any pre-scaler and the freq. is 31.7kHz, will it still work (given that the driver only supports 20kHz)?

Cheers!

7

I guess not, if it says not. I wasn't sure what you meant by "driver" in this context.

3.92 KHz should be a lot better (although audible, possibly).

8