PWM not working?

**Russ Kincaid** · - 25th October 2010, 16:14

This program runs (produces pulses of varying width):

Code:

REM DEVICE = 12F675
    CMCON = 7           'SETS DIGITAL MODE
    ANSEL = 0           'GPIO.0 TO GPIO.3 SET AS DIGITAL
    TRISIO = 0          'ALL OUTPUTS BUTGP3 (PIN4) IS INPUT ONLY
    VRCON.7 = 0         'TURN OFF VOLTAGE REFERENCE TO SAVE POWER
    DEFINE OSCCAL_1K 1  'TO SAVE OSCILLATOR CALIBRATION
        tme var word    'BEGINNING NUMBER OF PULSE CYCLES(COUNT DOWN)
        d var byte  'DUTY CYCLE OF PULSE, 127=50%
        j var word  'NUMBER OF CYCLES OF PULSE
    START:
       
         FOR d = 50 TO 250
         PWM GPIO.0,d,1
         PAUSEUS 500
         NEXT D
        
       GOTO START 
        end

This program does not:

Code:

REM DEVICE = 12F675
    CMCON = 7           'SETS DIGITAL MODE
    ANSEL = 0           'GPIO.0 TO GPIO.3 SET AS DIGITAL
    TRISIO = 0          'ALL OUTPUTS BUTGP3 (PIN4) IS INPUT ONLY
    VRCON.7 = 0         'TURN OFF VOLTAGE REFERENCE TO SAVE POWER
    DEFINE OSCCAL_1K 1  'TO SAVE OSCILLATOR CALIBRATION
        tme var word    'BEGINNING NUMBER OF PULSE CYCLES(COUNT DOWN)
        d var byte  'DUTY CYCLE OF PULSE, 127=50%
        j var word  'NUMBER OF CYCLES OF PULSE
    START:
       
         FOR d = 250 TO 50
         PWM GPIO.0,d,1
         PAUSEUS 500
         NEXT D
        
       GOTO START 
        end

Can anyone explain why this does not run (produces no pulses).

**LinkMTech** · - 25th October 2010, 16:25

Hi Russ,
Try this:

Code:

FOR d = 250 TO 50 STEP -1

**Acetronics2** · - 25th October 2010, 16:45

Hi, Russ

One thing hurts me in your program ...

you write :

produces pulses of varying width

in fact the PWM command produces pulses BURSTS whose total duration match the requested "duty" ratio ...

understand signal is not a nice pulse with one High level and one low level, but some kind of garbage.

No problem, if you drive a lamp or generate a voltage through a Low Pass filter ...
but not so good if driving some kind of inductive load ( motor i.e. )

in this last case ... you should create some Software PWM by regularly toggling a pin ... or use a 12F683 that has a HPWM module.

Alain