10KHz-50KHz frequency using CCP in '46K22

[jvd]

I'm trying to write a program to generate frequencies from 20KHz to 50KHz using CCP. The problem is resolution. I need 0.1KHz steps but the best I get with a prescale of 1 and around 50 KHz is 0.7KHz. How can I solve this?

Thanks

[Mike Walne]

You don't tell us your clock frequency.

At 50kHz (20us) you're wanting resolution of 1 part in 500 (0.2%)
With a 40MHz clock you should have 25ns resolution for the PWM module.
That's 0.125% resolution.
Should be good enough.
Or am I missing something?

Mike

[asmboy]

use the code in this post but change to an 8mhz clock and you have better resolution than you need.

http://www.ccsinfo.com/forum/viewtopic.php?t=50121

[jvd]

Thank you both.

[Ttelmah]

He is not going to get the resolution he wants. Look for a chip with the programmable frequency generator(best) or the power PWM (slightly better than the standard PWM).

Problem is that the PWM is not designed as a variable frequency generator. The PWM _period generator_ source clock basically only has a 8bit resolution, with three prescalers (so potentially about 9.5bits). If he runs it off the Fosc/4 (fastest clock), then at 40MHz, the available frequencies, jump in as:

PR2=198 F=50251.256
PR2=199 F=50000
PR2=200 F=49751.24

Better than he is dealing with (presumably a slower master clock), but still jumps of over 250Hz. Then of course with the high master clock, he will only get down to 39062.5Hz, and then changing the prescaler to Fosc/16, gives at 20KHz:

PR2=123 F=20161.29
PR2=124 F=20000
PR2=125 F=19841.26

So the PWM, won't give him 100Hz resolution, but much better than his 0.7KHz...

However the power control PWM module, has a 12bit PTPER register, and this allows you to not have to change the prescaler and at the 20KHz frequency from 40MHz would then give:

PTPER=499 F=20000
PTPER=500 F=19960.07

Better than required.

However the step size at 50KHz is still the same.

Using 20MHz and the standard PWM, Fosc/4 would then get down to just under 20KHz:

PR2=249 F=20000
PR2=248 F=20161.29

So a step size of 161.29 Hz at this frequency, but at 50KHz:

PR2=99 F=50000
PR2=100 F=49504.95

So nearly 500Hz at this frequency.

[jvd]

Thank you very much Ttelmah.
I'll try to see if power PWM is good enough.
Do you have any suggestions for a chip with the programmable frequency generator?

[Ttelmah]

Unfortunately at the moment I think this is only available on a couple of the PIC16's. Chips like the PIC16F1719.

Other comments also apply though - the faster the master clock, the smaller the step. Your PIC can go to 64MHz, and if you ran at (say) 50Mhz, and switched the prescaler as needed, then you could get:

PR2=249 F=50000
PR2=250 F=49800

/4 prescale
PR2=155 F=20032
PR2=156 F=19904

Switch to a PIC24, and several (most) have 16bit PWM's that can run off Fosc/2, with PLL's supporting in some cases Fosc up to 140MHz. Simpler since you don't then need to switch pre-scalers.