4 x PWM outputs

Hi Guys,

Been a while since I've dabbled in pic programming, but now need some advice and suggestions for a project I'm toying with.

I'm looking at developing a 4 channel dew heater controller for use with a telescope. Basic requirement is 4 separate PWM outputs, each controlled by a analogue pot. The frequency of the PWM would be a low 1Hz with the pot controlling the width of the pulse. Any suggestions of a suitable PIC,

Hi,
At such low frequency you can easily get away with a software based PWM scheme which means basically "any" PIC with atleast 8 I/O out of which 4 are analog will do. If you search the forum for Darrels SPWM routine it might give you kickstart in the right direction. Have a look Here for example.

Henrik, thanks for the suggestions and link. It's been upwards of 18 months since I last programmed in PBP - rusty ain't the word !

Code:

---

FOR LoopCount = 1 TO 4              ; Reapeat 4 times
        FOR DutyVar1 = 0 TO 150        ; Fade LED1 UP
            PAUSE 10
            RANDOM  RandVar           
            DutyVar3 = RandVar & $3F    ; Give LED3 a random dutycycle
        NEXT DutyVar1
       
        FOR DutyVar1 = 150 TO 0 STEP -1 ; Fade LED1 Down
            PAUSE 10
            RANDOM  RandVar
            DutyVar2 = RandVar & $3F    ; Give LED2 a random dutycycle
        NEXT DutyVar1
    NEXT LoopCount

---

If I follow the example above all the processing (for want of a better word) is done in a loop, so once DutyVar1 has had its value written to the port, it then moves on to DutyVar3 and does the same and then to DutyVar2, then loops back round and does then runs through the loop again adding or subtracting (or inserting a random value), then loops back .... etc etc.

Is there a way of having all three LEDs running in independent loops so that led1 doesn't have to wait for led3 to be actioned before having the next value written to it ? Or are we entering the realm of your PID routines that Darrel used to help me develop my 4ch reptile thermostat ??

Cheers

Malcolm

Hi Malcolm,
Using Darrels SPWM routines is probably easier but basically you can do something like this. (This is untested, meant to show the theory only)

Code:

---

Duty1 VAR BYTE    ' 0-255 = 0-100%
Duty2 VAR BYTE
Duty3 VAR BYTE
Period VAR BYTE

LED1 VAR PortB.0
LED2 VAR PortB.1
LED3 VAR PortB.2

Period = 0

PWMStart:
  ' Turn on outputs only if duty is more than 0
  If Duty1 > 0 THEN
      LED1 = 1
  ENDIF

  IF Duty2 > 0 THEN
      LED2 = 1
  ENDIF

  IF Duty3 > 0 THEN
      LED3 = 1
  ENDIF

PWMLoop:
  ' Increment period
  Period = Period + 1
 
  ' Time to turn off output 1?
  IF Period = Duty1 THEN
      LED1 = 0
  ENDIF

  ' Time to turn off output 2?
  IF Period = Duty2 THEN
      LED2 = 0
  ENDIF

  ' Time to turn off output 3?
  IF Period = Duty3 THEN
      LED3 = 0
  ENDIF

  ' Go and do other stuff needed
  GOSUB DoOtherStuff

  IF Period = 0 THEN
      GOTO PWMStart
  ELSE
      GOTO PWMLoop
  ENDIF

END

'--------------------------------------------------------------
DoOtherStuff:
' Do the ADC stuff etc here.

PAUSEUS 4000    ' Adjust to get desired PWM frequency.
'--------------------------------------------------------------

---

/Henrik.