3 channel PWM with customize duty cycle

[HenrikOlsson]

Hi,
I've tried it here, on a real physical PIC, and it works just fine.

I see that you've removed the Pause statement I had in there. If you don't have a delay there it will run thru the different dutycycles much faster then the actual PWM frequency. It tries to update the dutycycle registers several times during each PWM period which obviously won't work properly.

Your PWM frequency is 5kHz so if you really do want to see "all the steps" you must have atleast 1/5000=200us between updates. I measured the execution time of the loop to around 25us with a 20Mhz oscillator so you're basically trying to update the dutycycle 8 times every PWM period.....

[photoelectric]

yes, i remove the pause statement because i think adding it causing the pwm duty longer then the original pwm duty, hence changing my pwm pattern.
can we actually pause below 1 ms since i know pause statement can only be use for minimum 1ms right??
correct me if im wrong

when you tries on the real physical PIC, do you enable the pause statement??
i will try on real physical PIC later and feedback to you tmrw.

photoelectirc

[HenrikOlsson]

Hi,
The Pause statement won't effect the PWM since it's generated in hardware. If you have Pause 1 you'll basically get 5 periods of each dutycycle value.

You can Pause with microsecond resolution by using PauseUs (again, it's in the green book....)

When testing I used 300us and you can see the result in the attached screenshot. This is not a simulation, it's captured on real hardware.

[photoelectric]

hi, yes you are correct, i test it on real physical and it works fine.
But i dint really understand on how you calculate the execution of my FOR NEXT loop. Can you explain a liltle bit detailed on that.
below is the previous quote on how you calculate the execution time.

Your PWM frequency is 5kHz so if you really do want to see "all the steps" you must have atleast 1/5000=200us between updates. I measured the execution time of the loop to around 25us with a 20Mhz oscillator so you're basically trying to update the dutycycle 8 times every PWM period.....

[HenrikOlsson]

Hi,

I didn't calculate it, I measured it - as it says in the message ;-)

I put a TOGGLE PortC.0 just before the NEXT statement a put the scope on the pin. I got a 20kHz square wave which, because the pin is toggled each time thru loop, means it executes roughly 40000 times per second. 1/40000=25us

[photoelectric]

hi,
thks for the explanation, now my pwm works fines.
attached is my currently pwm pattern for ccp1 and ccp2.
NOw, i want to shift the CH2 pwm about 1/3 of the duration of the CH1 pwm which is

10.20ms/3 = 3.4ms

so, i intend to put pause command at:

Main1:
'For duty1 = 0 TO 44

'Go into the table AND retrieve the value pointed at by Duty1 AND put that value in Duty2
LookUp2 duty1, [0,70,137,208,275,341,408,470,529,588,643,694,745,7 88,827,866,898,925,_
953,968,984,996,1000,996,984,968,953,925,898,866,8 27,788,745,_
694,643,588,529,470,408,341,275,208,137,70,0],duty2



'Duty2 now contains the value from the lookup table that Duty1 points at. First time thru the loop it's 0 second time it's 18 and so on.

CCP1CON.4 = duty2.0 ' Setup 10-bit duty cycle as
CCP1CON.5 = duty2.1 ' a 10-bit word
CCPR1L = duty2 >> 2

Pause 4
CCP3CON.4 = duty2.0
CCP3CON.5 = duty2.1
CCPR3L = duty2 >> 2

PauseUs 210 'Use each dutycycle value 10ms before going to the next one.
'Next duty1
'PauseUs 100

GoTo Main1

but seem like ccp1 and ccp2 pwm duration increase above 10.20ms.
SO, what should i adjuzt to make pwm CH2 shift 1/3 of the pwm CH1
without increasing the duration?

thks
photoelectric

[HenrikOlsson]

Hi,
If you're trying to get the 120° phase shift you can't do that with a PAUSE. You have to remember that only the PWM is done by hardware, everything else, ie. the lookup and setting the dutycycle registers are done in software.

If you put a Pause 4 in there the whole software loop will "hang" for 4ms and whatever the CCP module is outputting will keep getting output untill it gets updated again. The FOR-NEXT loop will NOT run while the pause statements executes.

If you want 120° phase shift you could do that thru an offset into the loopup table. If the table is 45 entries you want entry 0, 14 and 29 the first time. Then you want 1, 15 and 30 and so on. I'd probably skip the FOR-NEXT loop and rewrite it to so that I'd have three counters, all counting from 0 to 44 but with an offset of 15 between them, these will then get the dutycycle from the table and assingn them to the proper CCPModule.

/Henrik.

[photoelectric]

Hi,

Im still trying to understand your explaination.
So i have to use count command?
can you give me an example of code for me to refer to?

photoelectric

[HenrikOlsson]

Hi,
I'm sorry to say this but it sounds like you need to take a step back and work thru the basics of programming with PBP. We can keep giving you code here until we've basically written the whole thing for you and you still wouldn't understand how it works.

Pause does just that - it pauses - no other code (except if you're using interrupts) is running while the Pause exeuctes. Like this:

Main:
  High PortB.0
  Pause 100
  Low PortB.0
  Pause 100
Goto Main

This will blink a LED connected to PortB.0 at 5Hz. If you put another pause in there, like:

Main:
  High PortB.0
  Pause 100
  Low PortB.0
  Pause 100
 
  Pause 1000
Goto Main

It won't blink at 5Hz because you just told it to pause for another second. The same thing happens with your 4ms pause. The FOR NEXT loop that is supposed to keep running and feeding the PWM module with a new dutycycle evert 210us will stop - completely - for the 4ms duration. But because the PWM is generated by hardware it will keep outputting whatever dutycycle it was last told to do.

The count command is used to count pulses on a pin, I don't think that is what you want?

The following is ONE way of doing it. This compiles fine but I have NOT tested it.

Phase       VAR BYTE[3]       ' Array used as pointers into lookuptable.
DutyCycle   VAR WORD[3]       ' Array storing the dutycycles retreived from the table
Temp        VAR WORD          ' Temporary variable to avoid hard to understand array indexing
i           VAR BYTE          ' General purpose counter
 
' The lookup table has 186 entries long, to get 120° phase shift we need to
' "start" the second phase at 1/3 of the cycle and the third phase at 2/3
' of the table. 186/3=62 so first phase starts at 0, second phase at 62
' and third phase at 123.
' Initilise pointers.
Phase[0] = 0 : Phase[1] = 61 : Phase[2] = 123
 
Main:
  Gosub GetDuty               ' Retrieve the dutycycle values for all three phases
  GOSUB SetDutyCycle          ' Set the three PWM modules accordingly
 
' Now increment the individual table pointers and make sure they wrap
' around to zero when they reach then end of the table. That way they
' will always stay 62 "steps" (120°) from each other.
 
  For i = 0 to 2
    Phase[i] = Phase[i] + 1
    If Phase[i] > 185 then Phase[i] = 0         'When pointer is > 185 we need to wrap around to 0.
  NEXT
 
  PauseUs 300
 
Goto Main
 
 
SetDutyCycle:
    ' Get value from the array of dutycycles and put it in the dutycycle
    ' registers of the 3 PWM modules. We could have used the array directly
    ' but this way (using a Temp variable) is easier to understand.
 
    Temp = DutyCycle[0]             ' Get dutycyle for phase 1 from the array and store in temp.
    CCP1CON.4 = Temp.0              ' Set the LSB's  
    CCP1CON.5 = Temp.1  
    CCPR1L    = Temp >> 2           ' Set the 8 high bits 
 
    Temp = DutyCycle[1]             ' Same procedure.
    CCP2CON.4 = Temp.0  
    CCP2CON.5 = Temp.1  
    CCPR2L    = Temp >> 2
 
    Temp = DutyCyle[2]              ' Same procedure.
    CCP3CON.4 = Temp.0  
    CCP3CON.5 = Temp.1  
    CCPR3L    = Temp >> 2
RETURN
 
 
' ------------------------------------------------------------------------------
' ---- Subroutine to retreive the three dutycycle values from the table.
' ---- Values will be stored in the DutyCycle array.
' ------------------------------------------------------------------------------
GetDuty:
' This For-Next loop runs three times. Each time it gets the value from the lookuptable
' that Phase[i] is pointing at. The Phase array pointers are incremented in the main loop
' and are always 62 "steps" appart so that a 120° phase shift is preserved.
 
For i = 0 to 2
  LookUp2 Phase[i], [0,18,34,52,68,85,102,119,137,152,170,187,203,220,236,253,267,285,302,_
     318,334,350,367,382,397,413,429,443,459,474,489,504,518,533,547,562,575,589,603,616,_
     630,643,655,669,681,693,706,718,729,740,752,764,774,785,795,806,815,825,835,844,853,_
     861,870,879,887,895,901,909,916,923,929,935,941,946,952,957,962,967,971,974,978,_
     981,984,987,990,992,994,996,997,998,999,999,1000,999,999,998,997,997,996,994,992,_
     990,987,984,981,978,974,971,967,962,957,952,946,941,935,929,923,916,909,901,895,_
     887,879,870,861,853,844,835,825,815,806,795,785,774,764,752,740,729,718,706,693,_
     681,669,655,643,630,616,603,589,575,562,547,533,518,504,489,474,459,443,429,413,_
     397,382,367,350,334,318,302,285,267,253,220,203,187,170,152,137,119,102,85,68,_
     52,34,18,0],Temp
 
 
     ' Lookup2 can't handle an array as the designator so we need to  
     ' put the value in a temporary variable first and then move
     ' it to the array of dutycycles.
 
     DutyCycle[i] = Temp
 
NEXT
 
RETURN

/Henrik.