SPI and digital potentiometers

buntay

Senior Member
Hello all.

I need a bit of help here cause i cant seem to grasp this. I would like to jump into playing with some digital pots and my question has 2 parts.

It seems you can only do SPI interfacing and I really don't understand whats supposed to be going on here, can somebody break the hspiout, and the shiftout(spiout) commands down in layman's terms and how to make this work with a digital pot? the manual doesn't seem to quite make sense to me.

The second question is lets say 1 pot with 256 taps isn't enough precision and I would like 2,3, or 4 pots that would give me 512,768,or 1024 taps and the same resistance range. could this be done? how?

Thanks to all in advance that help get me through yet another door in the micro-controller world.

F.
 

premelec

Senior Member
512K taps very hard to do if you want any accuracy - 2PPM - if you put one pot onto the output of a previous one it loads it and changes the range etc.. the loading effect is different for each previous step as the output impedance changes with a tapped string of resistances... lowering the effective output impedance of the first string with an amplifier introduces other errors... temperature coefficients also come in play - what are you actually trying to do? A commercial mechanical pot "Dekapot" with four dials may give you some ideas of how the resistances must be changed.
 

buntay

Senior Member
Thanks for the reply's so far. The link hippy provided are the pots I want to use. My application needs a 100k pot to control a motor speed...this is not a frequency driver and is irrelevant. It needs 100k pot to control it. Right now it uses a 10 turn mechanical pot and I would like to convert to digital so picaxe can run it. The problem lies in the speed range of the motor. 256 settings arnt enough, I need a higher resolution. I was thinking of controlling 2 - 50k digital pots in series.

Thanks again. :)
 
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