Can you use 2 PWM's simultaneously?

Hello Everybody. I need to output 2 PWM's at 15KHZ (approx) simultaneously and control the duty cycles independently. The PBP manual says you can do it on PIC's with more than 1 channel. However, the PIC I want to use is 16F684. According to the datasheet, the PWM module is multiplexed among 4 possible outputs. There was also a note that changing something while the PWM is running will "cause a glitch". Will this PIC work? I need a small PIC with more than 1 PWM and if I can't use the 'F684, do I have to move up to a 28 pin PIC? Thank you.

You DO need 2 CCP or ECCP modules ... an ECCP as the 684 just drive many pins from 1 CCP.

look for a PIC that has 2 CCP's ...

I think you need a 28 pin package ... at least. maybe 18F series in 18 pins: 18F1230/1330, if you find some !!!

Do a search on the µChip site for 2 CCP's ... you'll see what's possible.

Alain

see also here: http://melabs.picbasic.com/Scripts/perl/pic.pl

I'm not sure of the answer however there are two ways to produce PWM in PICBASIC. One is PWM the other is HPWM. HPWM is dependant on the pics hardware, however PWM is software based.

Couldn't you do one output HPWM and one PWM or both PWM?

Quote:

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Originally Posted by DynamoBen

I'm not sure of the answer however there are two ways to produce PWM in PICBASIC. One is PWM the other is HPWM. HPWM is dependant on the pics hardware, however PWM is software based.

Couldn't you do one output HPWM and one PWM or both PWM?

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Hi, Dynamoben

A soft 15 kHz PWM ...??? ...

Alain

I'm not sure if it would work or not. I haven't done much PWM work. I'm just throwing the topic out for discussion.

Thanks guys. Seems the smallest PIC with more than 2 CCP's has 28 pins. I just hate to waste about 20 i/o's - have them sit there and do nothing! As for the PWM command Ben, I want to use HPWM because once started, the HPWM constantly runs in the backround while the PIC is doing something else, until it's cancelled. This will drastically help me simplify the code.

I'm not sure what’s out there but how about using external circuitry to get the second PWM.

http://www.edn.com/article/CA305484.html

I have done this before. You just need to put the 2 PWM commands side by side in a for next and do it for one cycle. Adjusing the 3 that I have there will keep the same duty cycle for longer or shorter. Your eye can't tell the difference. This circuit would brighten with the push of one button and dim with the push of another. It would time out after the counter got to 1200 and they would both go out. Not energy efficient but this was one of the first programs I wrote for my thesis. It was on a 16f84a

DUTY VAR BYTE
sleeptime VAR WORD
i VAR BYTE

Input PORTB.0 'Decrease light and time button
Input PORTB.1 'Increase light and time button

DUTY = 127 'Set LED to half brightness
sleeptime = 0 'Set sleeptime to 0

START:
For i = 1 to 3 'Go through each duty cycle 3 times or 30ms in time
PWM PORTB.4,DUTY,1 'Output to LED
PWM PORTB.5,DUTY,1 'Output to LED
Next i

IF (PORTB.1 = 1) AND (DUTY > 0) Then 'Check to see if light decrease is pushed
Duty = Duty - 1 'Make LED dimmer by 1 count
EndIF

IF (PORTB.2 = 1) AND (DUTY < 255) Then 'Check to see if light increase is pushed
DUTY = DUTY + 1 'Make LED brighter by 1 count
EndIF

GoSub testsleep 'Check to see if sleep creteria is met; no inputs for delay time
GoTo START 'Return to start

testsleep:
IF (PORTB.1 = 1) AND (sleeptime = 1200) Then
sleeptime = 0 'Reset sleeptime if an input is high and sleeptime is already at max
duty = 127 'On input set the LED's to half brightness
EndIF

IF (PORTB.2 = 1) AND (sleeptime = 1200) Then
sleeptime = 0 'Reset sleeptime if an input is high and sleeptime is already at max
duty = 127 'On input set the LED's to half brightness
EndIF

IF (PORTB.1 = 0) AND (PORTB.2 = 0) AND (sleeptime < 1200) Then
sleeptime = sleeptime + 1 'Increment sleeptime if no inputs
EndIF

IF sleeptime = 1200 Then 'If sleeptime gets to 1200 turn off LED's
Duty = 0
EndIF

Return

Another idea, even if not the best, having a 2XHPWM frequency, fed the HPWM output on a INT pin, enable the interrupt, and play with the duty in the Interupt handler... in my head it's working but i know i miss something in it....

OR create your own with a Timer interrupt.. but geez, a little bit more tricky if you really need it 100% stable.

Or, a double I2C DAC could be intereting... something like MAX518 if you can deal with a 8 bit resolution. They're not too cheap but great and simple to use.

There's still the Digital pot way, or, or ,or... man it's incredible how many ways you can do it :o)


What is the purpose of those PWMs?

EDIT: Darrel already done a SSPWM solution bellow but the frequency range is probably out of what you need
http://www.pbpgroup.com/modules/wfse...p?articleid=12

Hi Steve. The purpose is for motor control. I already made a circuit using the pulsout command. It works but, if I can have HPWM running the motors in the backround, it frees-up the PIC to do other things like checking battery voltage, turn on lights, monitor the motor FET's for shorts, etc. I've pretty much decided I'll have to use a 28 pin PIC such as 16F73.