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I have been working mostly with 5V rated MCU but I am venturing into the world of 3.3V rated MCU's.

That should not effect PWM,right?

(PWM is bound to time, not voltage as I understand it)

Any info would be appreciated. Thanks.

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up vote 2 down vote accepted

The PWM is a discrete-value output that is modulated in time.

As such, it only requires a output that can encode two discrete steps. This output can be translated from one signaling format to another with no theoretical losses.

0V-5V is one common format, as is 0V-3.3V, and 0V-\$_{n}\$V, and more exotic current-mode mechanisms. However, translating from one signaling mechanism to another is possible with little effort, just requiring the use of the proper buffering device, so no single PWM physical coding mechanism is limited to that mechanism by anything other then the costs of the parts required to perform the level/mode translation.

So the answer to your question, on the face of it, is no.

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Thanks for the reply. By saying "buffering device" is it only buffering/reproducing the same pulse at a target level? – Bertus Kruger Mar 9 '14 at 23:18
@BertusKruger - Yes. For example, if you have a 0-3.3V signal, and you feed it through a buffer that outputs 5V if the input is greater then \$\frac{3.3}{2}\$V, and -5V is the input is less then \$\frac{3.3}{2}\$V, you now have a output that is basically identical to a device that outputs a -5 - +5V PWM pulsetrain natively. – Connor Wolf Mar 10 '14 at 0:22
Note that in real-world situations, you need things like hysteresis to prevent buffers and such from oscillating at the transition point, but that is more of an implementation detail. – Connor Wolf Mar 10 '14 at 0:23
I'm somewhat simplifying the answer here, since there are things like PWM systems that can use multiple voltage levels (or reverse polarity, ex: for driving motors in both directions), but that is really PWM plus something additional. There are also limitations on real world hardware with regard to how narrow pulses can be, and so forth, but things like that are implementation details. – Connor Wolf Mar 10 '14 at 0:25

The voltage of the PWM output pulses will depend on the processor's supply voltage. A processor operating on 3.3 volts can't produce a 5 volt output pulse.

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PWM is bound to time and not to amplitude other than it should have distinguishable and stable voltage or current levels.

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It depends on what you do with the generated PWM. If you use the PWM to a device that reacts to the duty cycle of the PWM then the result will be the same (assuming it accepts 3.3v input).

On the other hand if you use the PWM to generate an analog voltage then the result of a 5v PWM will be different compared to a 3v PWM. Of course in that case you can always use an amplifier stage to increase the PWM level to any level you want.

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