POT part values

[Michael]

I think (don't have the book handy), that POT performs best with specific values
for the RC network.

Something like a .1UF cap and 50K pot right?

So, if I stay in a certain "freq range" will I get the same result?

Like if I use a 500K pot, that's ok as long as I go down in capacitance?

Also --

How long does it take POT to sense the value? You have to stay in a loop?
For how long?

THANKS.

[Glenn]

Well, the manual is online..

http://www.melabs.com/resources/pbpm...8-5_53.htm#550

I usally try something that looks good, and then test if it works..

I didnt loop it, ofcoz there is some time it needs to read it, but it can't be very long.

Here is how I used it in my fancontroller:

main:
	pot portb.1,Scale,resist0	' read value resist0 with scale scale on port d.1
	LCDOUT $FE, 1, "Duty = ", #resist0, " ", #pwmfreq, "Hz"
	HPWM 1,resist0,pwmfreq
	pause 100
	GoTo main

The pause is mainly for the LCD, dont think it was needed for the pot command.

I used a 20K pot and a 220nF cap, with this setup a scale value of 145 was perfect.

[Michael]

Great -- I'll play with some cap values.

I'm stuck with a 500K trimmer in an exisiting design.

Hopefully, POT doesn't take to long to execute as my program needs to keep moving or I could have some problems.

Thanks.

[sayzer]

Great -- I'll play with some cap values.

I'm stuck with a 500K trimmer in an exisiting design.

Hopefully, POT doesn't take to long to execute as my program needs to keep moving or I could have some problems.

Thanks.

I guess, you will need to use 1nf for 500K pot to get the same linearity as 100nf with 5K POT.

__________________

[Acetronics2]

Hi, Sayzer

A Happy new year for the Two sides of the Bosphor.



No ...

You do have to stick to the 50k - 0.1µF @ 4Mhz harware config. other values will give much limited range and great loss of granularity and precision - Sorry Glenn, but dozen of tests have been run ...

POT doesn't work at all as RCTime ...

I think many threads have been launched on this sublect during the past years - including the POT mechanism description ...

Alain

[Michael]

Thanks.

A definitive answer indeed.

Guess I'm out of luck then. The cap is there for noise then?

[Glenn]

You do have to stick to the 50k - 0.1µF @ 4Mhz harware config. other values will give much limited range and great loss of granularity and precision - Sorry Glenn, but dozen of tests have been run ...

Well, what can I say, it works fine for me with 20K and 220nf.. I get around zero and 254 at the endstops, and linearity that is good enough for me, actually I just tried to pout the value on a darrel-bargraph (Yea, I do love them ) and it "feels" right when turning the pot looking at the bargraph.

Maybe its not ideal, but at least its good enough for me, and probably most people ?

..But if I can choose the value (In this case there was a pot inbuilt that I wanted to use) I'll think about it.

[Glenn]

Thanks.

A definitive answer indeed.

Guess I'm out of luck then. The cap is there for noise then?

Due to the manual its..

"The resistance is measured by timing the discharge of a capacitor through the resistor (typically 5K to 50K)."

And it really looks like its done this way too.