How to setup 38kHz PWM with PIC18F1330

[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