Adjusting PWM frequency

[skimask]

Is this due to my 4MHz XTal's or what?

Check out that PIC-Multi-Calc file. That should tell you if 4Mhz will get you the 1khz you desire.
Also keep in mind that there's a 'granularity' with everything in the digital world. I haven't run the numbers, but, just for instance, with a 4Mhz crystal, you might get .975khz and 1.025khz with just a single number change, simply because you can't divide by a fraction. Same reason you can't get EXACTLY 9600 baud from a 20Mhz crystal in a PIC. You'll get close, really close, but not EXACTLY.
Bump up Fosc and you might be able to get the numbers you want...

[flotulopex]

So this is what I did get from PIC MultiCalc.
<img src=http://www.picbasic.co.uk/forum/attachment.php?attachmentid=2413&stc=1&d=120548097 0>
Under "Possible match", the settings I use for 10 bts resolution are the same.

Now this question: why is the maximum duty value "1000"? A 10 bits resolution should allow 1024 steps, no?

[Bruce]

PR2 should be loaded with 249 for 1kHz @4MHz. Plug this into the calculation shown in the
data sheet for PWM resolution, and you'll see it's just short of 10-bits.

[flotulopex]

Bruce,

Why should it be loaded with 249?

If I calculate the possible resolution with this given formula, it makes exactly 10 bits with PR2 set to 255 "+1".
<img src="http://www.picbasic.co.uk/forum/attachment.php?attachmentid=2417&stc=1&d=120549250 2">
I have to admit that the PWM calculation is still a mystery to me; I only roughtly understand the different parameters involved.

But this is exactly why I would like to understand where this value "249" comes from.

Tosc is a fourth of one microsecond (using the 4MHz XTal), isnt 'it?

[Adrian]

Hi Roger
I'm pretty new to this as well but I'd like a shot at answering your question. If I'm wrong I get shot down as well!

Freq Calc

PWM freq = Fosc/(PR2 + 1) x 4 x Prescale
With PR2 at 249 this is 4,000,000/250 x 4 x 4 = 1000 Hz.

Your 255 gives 4,000,000/256 x 4 x 4 = 976.56Hz which is what you measured.

Resolution formula as you stated. With PR2 at 249 then Resolution is Log 4 x 250/Log2 = 9.9657 = 1000 bit resolution (999.94 actually)

With PR2 ar 255 then resolution will be Log 4 x 256/Log2 = 10 bit but your frequency then can't be 1kHz ...only 976 Hz

Hope I got this all OK guys

Adrian

[skimask]

Hope I got this all OK guys

Looks good to me...and it's also the reason why you'll never get exactly 1khz at 4mhz...

PR2 = 249 = 976.56hz
PR2 = 248 = 1004.0160642570281124497991967871 Hz

[flotulopex]

It's very clear now.

Thanks for your explanations.