Power Control PWM, HELP needed

[HenrikOlsson]

Hi,
OK, lots of questions....Lets take a step back and look at how the PWM module works - basically.
There's a timer (PTMR), a period register (PTPER) - these are common for all "channels". Then there are a 4 dutycycle registers and comparators.

The timer starts at 0 and counts up until its value equals that of the period register, it then starts over at 0 and it all repeats. If you load the period register with the value 150 the timer will count from 0 to 150 and then start over at 0, count to 150 and so on. If the timer is driven with a frequency of 1MHz it will count to 150 in 151us so the PWM frequency 6622Hz.

When the timer starts at 0 the PWM output is set. As the timer counts upwards its value is compared to that of the dutycycle register. When the two are equal the PWM output is reset.

So, if you load the period register with 150 and the dutycycle register with 50 you'll get a dutycycle of ~33% because the PWM period is divided into 151 steps. If you now change the value of period register to 499 the PWM frequency will change because it now takes 500us (instead of 151) to reach its destination and the dutycycle will change to 10% because there's now 500 steps in the cycle compared to 151 before. (This again asumes a 1MHz frequency being fed to the timer and is only meant to describe how the PWM generation works).

So, as you probably can see, the absolute value of the dutycyle register at which you'll get 100% dutycycle is equal to that of that of the period register.

This is basically how it works.

/Henrik.

[Kloney]

That explained A LOT !
now it all makes sense
thanks a lot for your help this couldn't be explained any better

[Kloney]

but shouldn't it be "Fosc/4" if I'm using (1:1) prescale ? (1MHz/4 in our case ) which gives PWM frequency 1655Hz ?

[HenrikOlsson]

Hi,
Note that I said - If the timer is driven with a frequency of 1MHz - which it would be if you run the PIC at 4MHz and select Fosc/4 as the PWM Timebase input.
Apart from that you are correct. If you'd run the PIC at 1MHz and set the PWM timebase to derive it's clock from Fosc/4 then the timebase will run at 250kHz and you'd get a PWM frequency of 1655.5Hz with the values in the previous example.

/Henrik.

[Kloney]

My bad
Once again Thanks Henrik

[Kloney]

Last two questions (I hope so )
in Bruce's Example he's multiplying pdcx by (Lowbyte,Highbyte) Low and high byte of what and what is the purpose of doing that ?
also
anyway I'm going to post the Code since I can't find the problem yet .. what I'm Trying to do is to generate a sinewave from pwm0 with an offset to use that later in a three phase inverter.. so I made this test which gives unexpected results
what I'm trying to do is using entries from a lookup table to generate pulses with duty cycles starting from 1000 to 2000 then back to 1000 (Positive half Cycle) after that going from 1000 to 0 and going back again to 1000 (Negative half Cycle) which isn't happening ..
I'm using 10MHz osc 18f4431, and a lookup table of 25 Entries (Which isn't enough for a smooth wave but this is for testing)

PORTB = 0
TRISB = %11000000 ' pwm0-5 output
PTCON0 = %00000000 '   (1:1) prescale , Free-Running mode 
PTCON1 = %10000000 'time base is on, Counting is up
PWMCON0 = %01000000 'Pwm0-5 enbbled for pwm output, pwm0-5 complementary mode
PWMCON1 = %00000001 
OVDCOND = %11111111  
PTPERL = $D0
PTPERH = $07     'PTPER = 2000
Main:
PDC0L = 00 
PDC0H = 10

PDC0L = 59
PDC0H = 12

PDC0L = 00
PDC0H = 15

PDC0L = 07
PDC0H = 17

PDC0L = 66
PDC0H = 18

PDC0L = 66
PDC0H = 19

PDC0L = 00
PDC0H = 20

PDC0L = 66
PDC0H = 19

PDC0L = 66
PDC0H = 18

PDC0L = 07
PDC0H = 17

PDC0L = 00
PDC0H = 15

PDC0L = 59
PDC0H = 12

PDC0L = 00
PDC0H = 10

PDC0L = 41
PDC0H = 07

PDC0L = 00
PDC0H = 05

PDC0L = 93
PDC0H = 02

PDC0L = 34
PDC0H = 01

PDC0L = 34
PDC0H = 00

PDC0L = 00
PDC0H = 00

PDC0L = 34
PDC0H = 00

PDC0L = 34
PDC0H = 01

PDC0L = 93
PDC0H = 02

PDC0L = 00
PDC0H = 05

PDC0L = 41
PDC0H = 07

PDC0L = 00
PDC0H = 10 

 GoTo Main

Thanks

[HenrikOlsson]

Hi,
You are currently updating the dutycycle register at a very fast rate, much much faster than the PWM frequency. Try insterting a short delay ( PAUSE 5 or whatever) between each duty cycle update. If that doesn't help try explaining what it does and what the problem is.

I'll have to look at Bruce's example to see if I can answer the other (first) question.

/Henrik.

EDIT: OK, looked at Bruce's example but I don't see where he's multiplying the pdcx with anything....could you clarify?