Edit - Arduino Stack Exchange

You are not logged in. Your edit will be placed in a queue until it is peer reviewed.

We welcome edits that make the post easier to understand and more valuable for readers. Because community members review edits, please try to make the post substantially better than how you found it, for example, by fixing grammar or adding additional resources and hyperlinks.

Required fields*

Increase PWM bit resolution

I would like to increase the PWM bit resolution of the Arduino Uno. At this moment it is 8-bit which I consider too low. Is this possible without losing the ability of interrupts and delays?

Koen

EDIT This setup delivers a 16-bit resultion

void setupPWM16() {
 DDRB |= _BV(PB1) | _BV(PB2); /* set pins as outputs */
 TCCR1A = _BV(COM1A1) | _BV(COM1B1) /* non-inverting PWM */
 | _BV(WGM11); /* mode 14: fast PWM, TOP=ICR1 */
 TCCR1B = _BV(WGM13) | _BV(WGM12)
 | _BV(CS11); /* prescaler: clock / 8 */
 ICR1 = 0xffff; /* TOP counter value (freeing OCR1A*/
}
/* Comments about the setup
Changing ICR1 will effect the amount of bits of resolution.
ICR1 = 0xffff; (65535) 16-bit resolution
ICR1 = 0x7FFF; (32767) 15-bit resolution
ICR1 = 0x3FFF; (16383) 14-bit resolution etc....
Changing the prescaler will effect the frequency of the PWM signal.
Frequency[Hz}=CPU/(ICR1+1) where in this case CPU=16 MHz
16-bit PWM will be>>> (16000000/8)/(65535+1)=30.5175Hz
*/
/* 16-bit version of analogWrite(). Works only on pins 9 and 10. */
void analogWrite16(uint8_t pin, uint16_t val)
{
 switch (pin) {
 case 9: OCR1A = val; break;
 case 10: OCR1B = val; break;
 }
}

Answer*

With some calibration you could sum the outputs of two PWM channels with differing weighting resistors. At the extreme you could notionally use one output to provide 8 bits of resolution and scale the other to 1/256th the level and add them so the 2nd channel covers one bit of range and you (again notionally) get 16 bits of resolution. Without immense care and adjustment all you'd get would be a mess. 
However by dividing the 2nd channel by 16 or 32 you could add several extra bits of PWM resolution. Just by adding 2 PWM channels analog filtered outputs you add an extra bit (as potential range is doubled for unchanged mV/bit). 
Notionally (again) for every additional divide by 2 you get an extra bit of resolution, but this can only be carried out for maybe 4 or 5 or 6 extra bits, with increasing accuracy requirements of scaling resistors and more difficult calibration and proneness to errors. 

Brief example. 
If one PWM is scaled down to give say 0 - 255 mV in 1 mV step, then summing two PWMs with equal amplitude would give a 0 - 510 mV range in 1 mV steps. 
If one PWM is scaled down by a factor of 32 then instead of adding 255 mV to the initial PWMs range it would add only 8 mV to the top end (0.256.32 = 8 mV, but resolution would be in 0.03125 (1/32nd) mV steps. 

While this could perhaps be achieved purely with resistor summing and RC filtering, use of an op amp summer would greatly improve results.

Also PWM ripple could be filtered with a simple RC filter but use of one opamp as a buffer (or even just a single transistor as an emitter follower) would give you 3 or 5 poles of low pass filtering and much better chance of achieving extra PWM resolution. I have not inspected the "phase coherence" of the PWM outputs but expect that they move in relative lockstep so you don't get the smoothing advantage of adding two uncorrelated waveforms.

More comment can be made if needed. Ask if interested.

Draft saved

Draft discarded

Edit Summary*

Cancel

This is clever! Seems that the Mozzi sound synthesis library uses this trick for it's so called "HIFI" mode.

Edgar Bonet
– Edgar Bonet

2015年06月17日 19:40:17 +00:00
Commented Jun 17, 2015 at 19:40
That is a great us of the PWM. But wouldn't this smooth out the waveform? I am asking this since you are using a RC filter. Haven't mentioned this in my question but I'm driving a DC motor with it <feeling ashamed>. Thank you for the input!

KoenR
– KoenR

2015年06月18日 06:14:14 +00:00
Commented Jun 18, 2015 at 6:14
@KoenR (fwiw: I don't see anything to be ashamed of.) I don't know what frequency response/rate of change you want in your ADC output. Or why you want N bits or how big is enough. Motors will usually not be usefully controlled by more than 8 bits - depends on how precision an application you have. Motor acts as part of a smoothing filter due to inductance. You need to say what sort of motor and how driven. And a circuit diagram is about essential. Unless motor is tiny you have a driver. A brushed motor fed PWM must have a a catch diode to pass motor current when PWM is off. Adding two ...

Russell McMahon
– Russell McMahon

2015年06月18日 09:01:07 +00:00
Commented Jun 18, 2015 at 9:01
... PWMs here is entiely doable but circuit details need to be known.

Russell McMahon
– Russell McMahon

2015年06月18日 09:01:34 +00:00
Commented Jun 18, 2015 at 9:01
Beware! In some cases, smoothing out PWM with a low pass RC is not desirable. E.g., if you plug the Arduino output into the gate of a MOSFET, the MOSFET will stay cold as long as it's driven by clean PWM. But if you smooth it out, the MOSFET will start dissipating much more heat. Sometimes that's not a good thing.

Florin Andrei
– Florin Andrei

2015年07月13日 06:46:03 +00:00
Commented Jul 13, 2015 at 6:46

Add a comment |

How to Edit

Correct minor typos or mistakes
Clarify meaning without changing it
Add related resources or links
Always respect the author’s intent
Don’t use edits to reply to the author

How to Format

create code fences with backticks ` or tildes ~
```
like so
```
add language identifier to highlight code
```python
def function(foo):
print(foo)
```
put returns between paragraphs
for linebreak add 2 spaces at end
_italic_ or **bold**
indent code by 4 spaces
backtick escapes `like _so_`
quote by placing > at start of line
to make links (use https whenever possible)

<https://example.com>

[example](https://example.com)

<a href="https://example.com">example</a>

formatting help »
answering help »

How to Tag

A tag is a keyword or label that categorizes your question with other, similar questions. Choose one or more (up to 5) tags that will help answerers to find and interpret your question.

complete the sentence: my question is about...
use tags that describe things or concepts that are essential, not incidental to your question
favor using existing popular tags
read the descriptions that appear below the tag

If your question is primarily about a topic for which you can't find a tag:

combine multiple words into single-words with hyphens (e.g. arduino-uno), up to a maximum of 35 characters
creating new tags is a privilege; if you can't yet create a tag you need, then post this question without it, then ask the community to create it for you

popular tags »

lang-cpp