How to setup 38kHz PWM with PIC18F1330

[HenrikOlsson]

It's not really a parameter in that sense, it's more like a side effect of the relationship between the oscillator frequency and PWM frequency. The closer they are in relation to each other the less resolution you get - it's just the way it works.

Basically, all it is a counter driven by the oscillator. The PWM period is set by telling a comparator at what count to "start over". If the counter is clocked at 1MHz and you tell it start over at the value 100 then the PWM frequency would be 1000000/100=10000Hz. If you tell it start over at 10000 the PWM frequency becomes 1000Hz and so on. This is what the PTPER value does.

The PWM output is set at the start of the PWM cycle and reset when the counter equals the dutycycle value. Now I Think you can see that when the PWM frequency in the example above is 1000Hz ther resolution will be 10000 "steps" while when the PWM frequency is 10000Hz the resolution will be 100 "steps". It's just the way a digital PWM generator works.

As for your ripple, make sure you have decoupling capacitor(s) as close to the supply pins of the PIC as possible. If the PIC has more than one pair of supply pins make sure they are all connected and properly bypassed.

/Henrik.

[flotulopex]

Thanks Henrik.

I had to use a 4,7µF capacitor to finally get a nice clean signal

Thank you for the explanation too. I'll have a go for some tests with the formulas; this is going to be the hardest for me...maths are not really my best friends

Roger

[flotulopex]

So there's where I am now:

Desired freq = 38kHz (PWMperiod = 26,3157µs), crystal (Fosc) 20MHz, prescaler = 1
SEARCHING FOR PTPER (max 4095)

Transform formula:
PWMperiod[µs] = (PTPER+1 * prescaler) / (Fosc / 4)
=> (PTER+1 * prescaler) = PWMperiod * (Fosc / 4)
=> PTPER+1 = (PWMperiod * (Fosc / 4)) / prescaler
(26,3157µs * 5'000'000) / 1 = 131

But how do I calculate the duty-cyle register (PDC0) admitting I'd like to have a 50% value to set?

[HenrikOlsson]

Hi,
There will be aproximately 4 times as many "steps" (+/-3) to the dutycycle value as you set PTPER to. If PTPER is 131 then you'll have 131*4 = 524 steps so to get a a 50% dutycycle you set the dutycycle register to 264 (PDC0H=1, PDC0L=8).

/Henrik.

[flotulopex]

Well, I just found this in the DS:

DS note, page 131:
To get the correct PWM duty cycle, always
multiply the calculated PWM duty cycle
value by four before writing it to the PWM
Duty Cycle registers....

I think this means a "100%" (in my mind, this is a signal always high - not useful but for the theory) duty-cycle value would be:
- PTER * 4 => 130 * 4 => 520

So, for a 50% duty-cycle, the PDC0 value is going to to be:
- 520 * 50% => 260

But, playing with the values, I can see on my scope that the numbers would be more accurate taking in account the PTER+1 (131 instead of 130) value.

Where is the truth?

[flotulopex]

...you were quicker