Inverting PWM signal

[e d]

Posted on January 23, 2018 at 21:51

Is there a quick and clever way of inverting a PWM signal in software (I am using timer 2 of the Discovery Kit board/STM32L100)? Was looking for the complementary feature but I don't want a pair of PWMs, just the inverted of the original signal (top).

Thanks in advance!

[T J]

Posted on January 23, 2018 at 23:22

there are three ways,

setup a separate PWM output to be as requested,

or use an inverter outside the processor,

or use a transistor

if you only need 1 output but inverted, there is a switch to do it in the cube, are you using the cube ?

[T J]

Posted on January 23, 2018 at 23:46

If you have tried to use the invert function and it doesnt work,

then you need either to read the pin and invert it to another pin in a a very tight loop,

or use an inverter.

otherwise you could invert the periods, so the 'on' time is 'off'

[Tesla DeLorean]

Posted on January 23, 2018 at 23:48

PMW mode 1 and mode 2 have opposite senses as I recall. 

[e d]

Posted on January 23, 2018 at 23:34

As I mentioned I was looking for a software solution. Yes I only need 1 PWM (inverted) output. What 'switch' in the Cube are you referring to? This:

sConfigOC.OCPolarity = TIM_OCPOLARITY_LOW;?

I've tried this (changed from

TIM_OCPOLARITY_HIGH) 

and it worked but when inactive the signal was low instead of high as seen in the 'Desired' trace.

Thanks

[e d]

Posted on January 24, 2018 at 00:02

That was what I was looking into but my scope is in repair now so I hope to get it back tomorrow to do some experiment with the PWM mode setting. I might have some follow up tomorrow that needs some feedback. Thanks

[e d]

Posted on January 24, 2018 at 18:57

Been playing with the OCxM (PWM mode) and the polarity and got the same result: looks good while active (inverted polarity as desired). But I couldn't seem to make the signal HIGH when inactive (PWM OFF time). Is it even possible in single output mode? I know in complementary pair it is possible (make dead band zero).

[waclawek.jan · Accepted solution]

Posted on January 24, 2018 at 21:19

How do you achieve that PWM OFF, in terms of TIM register values?

If it means TIMx_CCER.CCxE=0, try to switch on pull-up on given signal, if its loading is low enough. Otherwise, leave TIMx_CCER.CCxE=1 and set TIMx_CCMRx.OCxM to one of the 'force' levels (depending on value of TIMx_CCER..CCxP).

(Please spare me with any reference to  'libraries'.)

JW

[e d]

Posted on January 24, 2018 at 21:41

Yes, the CCxE (or CC1E specifically) = 0 for PWM OFF period (for PWM ON, 

CCxE 

= 1). Do you suggest going back and forth between GPIO mode (pull-up) and normal PWM mode? This has been discussed internally but sounds kludgy. I was looking for an elegant way of doing it. Otherwise it will be done in hardware.

The load on this signal is about 100 mW.

[waclawek.jan]

Posted on January 24, 2018 at 21:57

Do you suggest going back and forth between GPIO mode (pull-up) and normal PWM mode?

No.

This has been discussed internally but sounds kludgy.

Regardless of the 'no' above, why?

And with regard of 'No', as I've said above: leave TIMx_CCER.CCxE=1 and set TIMx_CCMRx.OCxM to one of the 'force' levels .

JW