PWM

[AndrewB]

Is it possible to.
Output a pulse train that is spread out evenly in one cycle and is equal to the duration of a PWM cycle.

[David John Barker]

Can you give a little more detail?

[AndrewB]

As you know PWM is split into a hard duty cycle.
On and off.
for example the PWM signal at one point in time is 50/50.
50% on 50% off in the cycle.

I would like to spread that 50% of the on section in fine pulses over the whole cycle.
So the percentage on is the same but spread out of the whole cycle.

the frequency I'm looking at using 240Hz .

[David John Barker]

The PIC's hardware on / off times are based on frequency. So if you have a duty cycle of 50%, there are typically many on /off pulses a second and not just a simple on for 50%, then off for 50%. So you should get a spread of pulses in the whole cycle. I may have misunderstood you though. If hardware PWM isn't going to work for you then you can use a hardware timer based approach, which may be more suitable to your needs (that is, you can tweak to meet your specific needs). You can get pretty good results this way at relatively low frequencies (with lowish bit resolution) and you don't require too many channels. Something like:

Code: Select all

' the pwm frequency (in Hz)
' high value -> fast refresh, more CPU cycles
' low value  -> slow refresh, less CPU cycles (may flicker)
private const _pwm_frequency = 120                                       

' now calculate the timer reload value, given the pwm frequency
private const _fosc as single = _clock * 1000000 / 2                   ' instructions per second               
private const _update_frequency = _pwm_frequency * 256                 ' actual frequency (pwm * steps)
private const _reloadTimerValue as ushort = _fosc / _update_frequency  ' reload value

' this event handler is triggered when a timer interval reaches zero...
dim pwmDutyCycle as byte = 0
dim pwmPin as D3
dim pwmCounter as byte = 0
private Interrupt OnTimer(Pic.T1Interrupt)
   if pwmCounter = 0 then
      high(pwmPin)
   elseif pwmCounter = pwmDutyCycle then
      low(pwmPin)
   end if         
   pwmCounter += 1
   IFS0.3 = 0
End Interrupt

to set the duty:

Code: Select all

sub SetDuty(pValue as pwmDutyCycle)
   T1CON = 0                   ' clear Timer 1 - no presccale
   PR1 = _reloadTimerValue     ' set timer period
   IFS0.3 = 0                  ' clear timer interrupt flag
   T1CON.15 = 1                ' switch timer on   
   enable(OnTimer)             ' enable the interrupt handler
end sub

[AndrewB]

Tried this and it gives a normally PWM.

Am I correct in just adding

Code: Select all

Sub Main()
     setduty(50)
End Sub


What I was looking for in one cycle is shown in the attached png.
top is a normal PWM cycle below is what I would like.
I have also emailed you a pdf from Cypress.

[David John Barker]

I don't think I can be much more help, the two methods I have previously described (hardware or software through timer) have always met my PWM needs. If a PIC isn't up to the job then perhaps take a look at a third party chip, like the one you mentioned - please do report back with your results though...

[AndrewB]

Ok Thanks David. I will see what this Cypress cpu can do.

[Jerry Messina]

If the top waveform in the picture is 240Hz, then the bottom one is 6x-7x that... about 1.4KHz.
Also, the bottom trace isn't a constant PWM frequency.

Are you trying to dither/modulate the PWM signal, or is it just poorly drawn?

[AndrewB]

Bad sketch.
Yes dithering the PWM.

[David John Barker]

How did you get on with your dithering PWM? Did you make any inroads?