Quick n dirty HPWM out of phase [Archive]

ChrisHelvey

- 25th July 2007, 04:11

Assuming a PIC with at least 2 PWM channels, can someone suggest a quick way to output two PWMs exactly 180 degrees out of phase with each other?

I can do it by turning alternating pins on and off but there's no CPU left to do anything else without messing up the waveform.

Trying not to recreate the wheel...or maybe it's so obvious I can't see the answer.

Thanks

Archangel

- 25th July 2007, 06:47

Assuming a PIC with at least 2 PWM channels, can someone suggest a quick way to output two PWMs exactly 180 degrees out of phase with each other?

I can do it by turning alternating pins on and off but there's no CPU left to do anything else without messing up the waveform.

Trying not to recreate the wheel...or maybe it's so obvious I can't see the answer.

Thanks
Hardware ways, flip flop, inverter, transistor . . . since you do not want to burden PIC.

Darrel Taylor

- 25th July 2007, 08:16

Use a PIC with an ECCP (ENHANCED CAPTURE/COMPARE/PWM MODULE)
16F690 is one.

Put it in Enhanced Half-Bridge mode.
Outputs will be on P1A and P1B pins.

keithdoxey

- 25th July 2007, 10:30

Assuming a PIC with at least 2 PWM channels, can someone suggest a quick way to output two PWMs exactly 180 degrees out of phase with each other?

How about a PIC with a Comparator. Feed the PWM into comparator and use the comparator output to give your out of phase signal.

Advantages...

No external components
no software overhead other than setting up comparator

Disadvantages...

You lose one comparator and a few pins on your PIC

Darrel Taylor

- 25th July 2007, 10:35

oooo, I Like it Keith. :D

keithdoxey

- 25th July 2007, 10:50

oooo, I Like it Keith. :D
 

Sadly I cant take the credit.

The idea was originally suggested by Melanie but it stuck in my mind because I have an device that uses the HPWM to generate an IR carrier which I then gate with a transistor. Too late to change my existing design but I will use the comparator to do the gating in a later revision of the board.

Not only does it save a transistor and a couple of resistors but has a better voltage swing as well!

Darrel Taylor

- 25th July 2007, 11:01

Darn Melanie. She's just too smart for her own "Britches".

Which is why I tend to envision her without them.

Oh wait, I'd have done that anyways. :)

At least you applied it to the right problem. http://www.picbasic.co.uk/forum/images/icons/icon14.gif

ChrisHelvey

- 25th July 2007, 20:54

That is a very elegant solution - just what I was looking for.
Thanks, I will look in to this. The link to Melanie's post is very helpful. Since 0 Volts is "low voltage" it should work exactly the same - when the pulse goes low, the comparator output goes high.
Brilliance is always in the simplicity...

ChrisHelvey

- 26th July 2007, 22:53

Just for the record. I followed through on this and the above idea (using the comparator) works great on a 16f877A with a 20Mhz crystal.
I just tied the output of the HPWM pin to the Comparator input. I struggled with setting the registers properly because that part is still a bit greek to me (read - I had to experiment until I got what I wanted and then look at the data sheet to figure out WHY it did what it did.)
The wave form is very sturdy and quite square all the way up to 15Khz. I even managed to figure out how to use one of the two ADC inputs the comparator shares on PortA.

I'm learning. As an old dog, SLOWLY, but I'm learning....

Thanks everyone,

Chris Helvey

keithdoxey

- 26th July 2007, 22:59

Just for the record. I followed through on this and the above idea (using the comparator) works great on a 16f877A with a 20Mhz crystal.
I just tied the output of the HPWM pin to the Comparator input.

Glad you got it working.

I struggled with setting the registers properly because that part is still a bit greek to me

Tip! - Download the English version of the Datasheet :) :)

Melanie

- 27th July 2007, 08:45

Darrel really! In exchange for a really big bag of negotiable money all in nicely used notes etc etc (pics, noddy currency and vapourware doesn't count), I'll send you a signed naughty photo... *smiles*

Darrel Taylor

- 27th July 2007, 13:13

Melanie

- 28th July 2007, 09:53

Ohhh... temptations...

Must say you are a wizzard constructing web pages though... Harry Potter would be proud...

As a 'big bag' full of loot, it's a bit lightweight... (new job obviously's paying well I see...) flight over to California for a trip to the cinema... ooohhh... bargain can't wait... let me rain-check that... *smiles*

Just curiosity, are you counting the hits on the Yes and No buttons?

Darrel Taylor

- 28th July 2007, 11:37

Awwww, Jilted again. :(

Good thing I'm a nerd. I'm used to it. :)

> As a 'big bag' full of loot, it's a bit lightweight...

Yup, should have used $1 bills. Would've filled a good sized duffle bag.
But you're right, the Jobs going good.

No, not keeping track of the clicks or anything.
After the fiasco with the Javascript LCD and Firefox. I'm trying to learn Flash to see if I can do the same kind of stuff, that'll work in any browser.

Of course, that means you all have to put up with my silly creations along the way. :D

scasale

- 17th August 2014, 05:29

The following is how I do it. works well and is super easy. I see this is an old post, but hey, it might help someone!
Use the tilda ~ before the variable! I use this on a 16F873A and a 16F1503 and the 1824, works great.

'*********** Define ADCIN parameters *************************************

DEFINE OSC 4
DEFINE ADC_BITS 8 ' Set number of bits in result
DEFINE ADC_CLOCK 3 ' Set clock source (3=rc)
DEFINE ADC_SAMPLEUS 15 ' Set sampling time in uS

XADCVAR VAR WORD ' X ADC Result
PWMOUT1 var byte ' PWM CH1
PWMOUT2 VAR BYTE ' PWM CH2

'*********** SET PORTS ************************************************

TRISA = %000001 ' Set PORTA
TRISB = %00000000 ' Set PORTB
TRISC = %00000000 ' Set PORTC
ADCON0 = 0 ' Set up ADCON1

BEGIN:

'*********** CHECK ADC VALUE ******************************************

MAIN:
ADCIN 0, XadcVar ' convert ADC value to a byte value:
PWMOUT1 = XadcVar ' X Axis
PWMOUT2 = ~XadcVar ' Y Axis

Hpwm 1,PWMOUT1,1600
Hpwm 2,PWMOUT2,1600
Goto BEGIN

end