Inverting the o/p of hardware PWM

[AndrewC]

I'm working up a project to use 3 hardware PWM modules. Probably on a 16F777. The PWM signal goes into the signal pin of a LUXEON Buckpuck. The trouble is that that pin thinks 5V is "OFF" and 0V is "ON" for the led. So a 100% duty cycle would have the led off all the time and vice versa for a 0% duty cycle.

I could just put a line in to subtract my desired duty cycle count from 1024 (if in 10bit mode) and invert it that way, but is there an easier way to just invert the output of the PWM module ?

[Darrel Taylor]

Short answer ... NO.

There isn't really a 10-bit mode. Depending on the frequency being used, there may only be 6-7 bits of resolution. Then instead of subtracting from 1024, you'd have to subtract from the maximum resolution.

If you have PBP2.50 which allows all 3 HPWM's. You could subtract from 255, because the HPWM command always scales it to the available resolution.

It could be done by modifying the Library, but you probably don't want to go there.

There's always the 74HC04.
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[AndrewC]

Thanks Darrel, it's only a couple of extra lines of code so not too difficult. 3 PWM modules - perhaps I wasn't being clear, I didn't mean three times HPWM, but using the three different embedded PWM modules in a PIC such as 16F777 and addressing them directly.

Andrew

[Darrel Taylor]

perhaps I wasn't being clear, I didn't mean three times HPWM, but using the three different embedded PWM modules in a PIC such as 16F777 and addressing them directly

Yikes,

Then maybe my answer wasn't clear either, cause that's what I was talking about as well.

I could be more specific, if you were too.

What frequency are you running the PWM at?
What OSC?
Which version of PBP?
PM or MPASM?
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[AndrewC]

Hi Darrel,

Basically following Bruce's example in this thread:

http://www.picbasic.co.uk/forum/showthread.php?t=1770

PBP: 2.46
20MHz
MPASM

Hopefully I'm not missing something obvious - it has happened before and will doubtless happen again A 16F777 has 3 PWM modules and I'll control them directly.

thanks, Andrew

[Dave]

AndrewC, Why not just use a transistor and a couple of resistors?

Dave Purola,
N8NTA

[Darrel Taylor]

Hi Darrel,
Basically following Bruce's example in this thread:
http://www.picbasic.co.uk/forum/showthread.php?t=1770

Oh,OK.
According to that, it's running the PWM at the absolute minimum frequency, which gives exactly 10-bit resolution.

From you're last post I see that it's not continuously changing the dutycycle. So I don't need to talk about "glitch-free" either.

So it looks like your original idea from post #1 is the way to go, with 1 exception.
Subtract from 1023 instead of 1024.
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