Fast PWM on Pins 9 and 10 of Arduino Uno

Projects General Guidance
1

I am currently implementing fast PWM mode on an Arduino Uno. I have the set up code attached. I am getting an output near 0.5 V which is expected from 0CR1A when set to 102. I downloaded the code into my spare Arduino and am getting an output near 0.67 V. Is the DAC (pwm) output on my spare burnt?

void setup() {  
  Serial.begin(9600);
  pinMode(pwmPin_sensor, OUTPUT);
  pinMode(pwmPinDash, OUTPUT);
  /* Set up 10 bit resolution PWM on arduino */
  TCCR1A =
      (1 << COM1A1) | (1 << COM1B1) |
      // Fast PWM mode.
      (1 << WGM11);
  TCCR1B =
      // Fast PWM mode.
      (1 << WGM12) | (1 << WGM13) |
      // No clock prescaling (fastest possible
      // freq).
      (1 << CS10);
  OCR1A = 102;
  OCR1B = 620;  // 18 Volts
  // Set the counter value that corresponds to
  // 1023 (10 bit)
  ICR1 = 0x03ff;
}
2

I don’t really understand your question completely, but the UNO doesn’t have a DAC, so that may be contributing to your disappointment.

Do you have an oscilloscope? You could look at the output pins to check the PWM waveform.

3

I know it does not have a true DAC. The PWM output was around 0.5 V, but now it seems to be at 0.67 V with the same input value for 0CR1A.

4

For how I have the timers and pwm set up, with an input of 0CR1A = 0, the output voltage should be ~ 0 Volts. For an input of 1023 (TOP value) it should be ~ 5 Volts. For an input of around 100, the output SHOULD be 0.5 V, but it is way higher. When I input 1023, the output is 4.34 Volts. I cannot even get an output of 5 Volts anymore.

5

Do you know what PWM is ?

The output voltage doesn’t change, it’s either 0 or 5V (Vcc) on and off - depending on the duty cycle you specify.

6

Yeah I do actually. Not very hard to make a low pass filter to turn pwm into a similar DC voltage

7

Even when the duty cycle is at 100%, the output should be a continuous 5 Volts. It is not.

8

Well, I guess it’s about time we saw your schematic.

9

Your code produces signals with duties 10.1 and 60.6% on my LA, which exactly matches your OCR1x values.
So look for your error.

10

How is this measured?

11

Where and how defined the pins?

12

A multimeter put on DC Voltage

13

Pins 9 and 10 respectively. Above the set up

14

It is also being read in by a nFR Microcontroller to test different points on a gauge.

15

Which Arduino is your spare? Nano has a reverse polarity diode in series with USBV that drops 0.3 ~ 0.5V so you only get 4.5 ~ 4.8V on 5V pin.

16

Not reading off of 5V Pin. They are both Arduino Unos.

17

I downloaded the code into Wokwi and am getting an output on pin 9 of 6.4μs.
The PWM period is 64μs.
So yeah, 0.5V out of 5V max.

Output on pin 10 is 38.75μs.

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18

Interesting. Wonder why the output is different on my spare...

19

See the post #9
I already wrote to you the same as @dlloyd: there are no problems with the code, it works correctly.

I think that your voltage measurements are wrong.

20

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