Arduino Uno PWM - Strange Results

Projects Programming
1

Hi Everyone,

I've been experimenting with the PWM Waveform Generation Modes on the ATMega328P. I've been getting some strange results and I can't figure out if its a problem with how I'm writing the firmware or how I'm interpreting the datasheet.

Here's the first piece of code that I wrote to emulate the analogWrite() function:

// Waveform Generation Mode 0
// Table 15-4 of the datasheet

void setup()
{
  DDRB = (1<<PB1); // set pin 9 as output
  
  TCCR1A |= (1<<COM1A1);
  OCR1A = 125;
}

void loop()
{
}

The above code produces an output of around 2.5V out of pin 9. The strange thing (for me) is that according to the datasheet, TIMER1 is a 16bit timer, so it should overflow at 65536 ticks. From what I understand setting OCR1A between 0 and 65535 will change the duty cycle of the pulse. So, having set the OCR1A at 125, shouldn't I be getting an output of around 0.01 V instead of 2.5V? The results seem to imply that the clock is overflowing at 255.

For my second foray into PWM land, I wanted to try and create a 2.5V signal using the ATMega's fast PWM mode. Here's what I got:

void setup()
{
  DDRB = (1<<PB1);
  
  TCCR1A |= (1<<COM1A1) | (1<<WGM11);
  TCCR1B |= (1<<WGM13) | (1<<WGM12) | (1<<CS10);

  ICR1 = 19999;
  OCR1A = 10000;
}

void loop()
{
}

I set ICR1 (the overflow value) arbitrarily to 20000 ticks then set OCR1A (the compare value) to about half that. I set Channel A to non-inverting mode, but (I think) it wouldn't have made a difference if I set it to inverting mode. When I flashed this onto the Arduino I was getting a steady (5-8V) out of pin 9, and I can't for the life of me figure out why.

I would appreciate any insight you can offer.

2

The above code produces an output of around 2.5V out of pin 9.

Better google PWM, pulse width modulation.
this is not a voltage!

3

A digital voltage meter will not accurately display the 'output voltage' of a pwm pin. It will display a number but it's pretty much meaningless unless the pwm is set to 0% or 100%. A scope is one instrument that will accurately display a pwm output signal. Typically it's not a common need to measure a pwm output signal, but maybe we are missing something?

4

Marek91:
I was getting a steady (5-8V) out of pin 9, and I can't for the life of me figure out why.

There is only one way to get more than 5V out of an Arduino pin: measurement error.

5

LarryD

LarryD:

The above code produces an output of around 2.5V out of pin 9.

Better google PWM, pulse width modulation.
this is not a voltage!

Hi. I was referring to the average voltage that an external load (e.g. a motor) would feel. Have a look here. Be sure to read the second paragraph.

6

Would you supply a schematic?

7

I verified with my scope, the first code chunk outputs a 49% duty cycle.

The second code chunk stays high all the time.