PWM - Dutycycle affecting frequency?

[flotulopex]

Hi,

I'm just playing around with PWM (...and PicMultiCalc).

While making some experiments, I notice that changing the dutycycle will affect the Period's duration (frequency).

First, I set dutycylce to 127 (=50%). The frequency is 92,24kHz.
<img src="http://www.picbasic.co.uk/forum/attachment.php?attachmentid=1640&stc=1&d=117939319 7">

Second, same test at 64 (=25%) dutycycle. The frequency is now 46,02kHz.
<img src="http://www.picbasic.co.uk/forum/attachment.php?attachmentid=1639&stc=1&d=117939319 7">

In my comprehension of PWM, dutycycle would affect only the ratio of the HIGH against the LOW signal.

Is this okay or am I measuring something wrong?

[mister_e]

i guess something is wrong in your code. Can you post it here?

[Acetronics2]

Triggering parameters and signal amplitude ... please !!!

Alain

[mister_e]

36 mVPP looks like the scope is set in AC mode.

[flotulopex]

So, here's the code:

' Fuses
@ DEVICE PIC12F683,FCMEN_OFF
@ DEVICE PIC12F683,IESO_OFF
@ DEVICE PIC12F683,BOD_OFF
@ DEVICE PIC12F683,CPD_OFF
@ DEVICE PIC12F683,PROTECT_OFF
@ DEVICE PIC12F683,MCLR_OFF
@ DEVICE PIC12F683,WDT_OFF
@ DEVICE PIC12F683,INTRC_OSC_NOCLKOUT

'-------------------------------------------------------------------------------
' Registers   76543210
ANSEL      = %00000000  'Disable analog inputs
ADCON0     = %00000000  'ADC is OFF
CMCON0     = %00000111  'Comparator is OFF
OPTION_REG = %10000000  'Pull-Ups disabled...
TRISIO     = %00000000  'All PORTBs are Outputs
GPIO       = %00000000  'All PORTBs are Low

'-------------------------------------------------------------------------------
' Defines

'-------------------------------------------------------------------------------
' Variables

'-------------------------------------------------------------------------------
' Program
MAIN:
    PWM GPIO.2,64,1000
    goto main
end

And here is how the oscillo is set:
<img src="http://www.picbasic.co.uk/forum/attachment.php?attachmentid=1649&stc=1&d=117941075 3">

[Acetronics2]

Hi,Flotul ...

Not good to change PWM parameters all the time !!! ( a re-write is a change ... )

DO NOT use PWM in such a tight loop !!!

And try it removing the loop ... will work fine,then !!!

Alain

[skimask]

MAIN:
PWM GPIO.2,64,1000
goto main
end[/code]

Try adding a 'DEFINE NO_CLRWDT 1' and see if the freq still changes, or changes as much.

PBP (by default) adds a ClearWDT instruction before every PBP instruction. So, when you get down to the smaller duty cycle, that instruction executes more and more often, affecting your PWM frequency.

The problem with the PWM command, being software driven, is that it's frequency is affected by everything else in the program as a whole. It doesn't matter how short you make the program (as you've made the program as practically short as it can get), it'll still be affected by the 'goto main', it'll even be affected if the 'goto main' jumps across a page boundary.

PWM driven from an interrupt or a hardware module is about the only way to go if you want to keep a stable base frequency.

There's nothing wrong with your program, or your setup, or your 'scope settings...ya just need a bit more understanding the logic that's running underneath the PWM command...