How does PWM work in the Arduino?

Question

The problem that I have is that I can't understand the working of PWM in Arduino, even though I totally understand what PWM is.

Like the Arduino has a 980hz refresh rate (~1ms clock speed) on two of its PWM pins and 490hz refresh rate (~2ms clock speed) on the others PWM pins.

Then there is Arduino's internal crystal oscillator which works at 16Mhz (~62ns clock speed).

And then I came across this answer:

Why change microcontroller's PWM frequency?

In this the accepted answer gives a little overview on how PWM works.

But I can't seem to understand how does PWM wave fits inside the input window of PWM pins.

As PWM pin take input in every ~1-2ms how are PWM waves formed inside that input window? Can someone explain by showing an example of working like the above answer did.

And does the clock speed of crystal oscillator have anything to do in this process?

*I am totally new to electronics

Answer 1

You might find the AVR131: Using the AVR’s High-speed PWM application note useful.

Let's be clear on a couple of points regarding PWM.

1. A fixed frequency pulse train is generated.
2. The pulse-width is modulated (adjusted) so that the on-time varies from 0% to 100% depending on the "analog" value fed to the PWM register.

Let's say you want a PWM with a resolution of 256 steps.

• The PWM counter is reset (to zero) at the start of each PWM cycle and the output is switched high.
• A counter starts counting at a rate determined by the PWM oscillator and configuration registers. This count value generates the ramp shown in Figure 2-2.
• When the counter exceeds the OCR threshold value the output is switched off (but the counter keeps on going to 256).
• On the clock pulse after 255 the counter rolls over to zero, it resets and the process starts again.

It should be clear by now that the maximum possible PWM frequency would, in this case, be CPU clock / 256 but it may be much lower due to other divisions, etc., in the PWM controller.

So, to answer your questions:

But I can't seem to understand how does PWM wave fit inside the input window of PWM pins.

The beauty of PWM is that the output is either fully on (high or 1) or fully off (low or 0). This makes it very easy to implement with digital logic. There is no waveform inside - only binary counters and binary comparators. It is the average value of the output that gives the analog output.

This can work digitally to, for example, dim the backlight in your phone. The backlight is switched between fully on and fully off quickly enough that you don't notice the flicker and the brightness appears to be proportional to the PWM duty cycle. For a true analog signal - a voltage varying between 0 and 5 V, for example, then a simple RC (resistor-capacitor) low-pass filter may suffice.

As PWM pin's take input in every ~1-2ms how are PWM waves formed inside that input window?

Hopefully that's clearer now.

And Does the clock speed of crystal oscillator has anything to do in this process?

Yes. Everything will be based on that but usually after having been divided down by some power of 2 (4, 8, 16, etc.).

Answer 2

As I read the question, I see repeated reference to "input window of a PWM pin." I may be misunderstanding that, but it makes me think that you may be thinking of PWM pins as inputs. They're not - they're outputs. They output a square wave of a varying duty cycle. You can turn that into analog by adding an RC low-pass filter.

You also talk about forming PWM waves inside of the input window. My reading of that suggests that you may be trying to make a complex waveform where the waveform changes more frequently than the PWM frequency. This is not possible. PWM output can be used to make a complex waveform, but the rate of change of that output is going to be limited by the low-pass filter that converts the PWM output into analog, and that's going to be related to the PWM frequency, which will also limit the granularity of the duty cycle.

If you want to make high speed complex analog waveforms, then you have no choice but to use a proper DAC instead. PWM outputs converted to analog can only really be used to create analog voltages that change very slowly.

Answer 3

You might want to take a look at timer section of the the datasheet of the microcontroller you have on your Arduino, it will explain how it works. But basically, if you have a 8-bit timer, there are 256 steps before it overflows. The timer will run on the crystal frequency, or some division of it, and will set the PWM output pin high when counter matches 0, and will set the PWM output pin low when counter matches user set value.