How to interface a rotary potentiometer with the propeller?

turbosupra · 2010-07-19 04:07

I'd like to interface (2) rotary position potentiometers with the propeller. They are 0-9 position potentiometers and I'd want the propeller to be able to interpret an integer from the varying resistance or voltage.

Anyone ever done this? Do you have a code and schematic example?

Mike Green · 2010-07-19 04:15

Set up the potentiometers as voltage dividers and use the ADC technique shown in the AN001 application note and its associated sample program.

www.parallax.com/tabid/832/Default.aspx

Gerry Keely · 2010-07-19 06:48

You could also·use an rc method as decribed on page 122 of the·Book - "Propeller Education Kit Labs: Fundamentals"(located at same location indicated by Mike).

Schematic given for this circuit is not quite correct in that a resistor is required between the pot wiper and the propeller pin to prevent a direct short on the pin when the pot wiper is at grounded end of pot.I have used a 330 ohm resistor in the past.

Gerry

Post Edited (Gerry Keely) : 7/19/2010 7:43:36 AM GMT

ErNa · 2010-07-19 07:07

I have done it, and code and schematic are somewhere in this forum. Search for Muxed ADC or something like that

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turbosupra · 2010-07-19 19:17

Is this it?

http://forums.parallax.com/forums/default.aspx?f=25&m=261327&g=269023#m269023

ErNa said...
I have done it, and code and schematic are somewhere in this forum. Search for Muxed ADC or something like that

turbosupra · 2010-07-19 19:23

Thanks Mike!

Mike Green said...
Set up the potentiometers as voltage dividers and use the ADC technique shown in the AN001 application note and its associated sample program.

www.parallax.com/tabid/832/Default.aspx

turbosupra · 2010-07-22 21:44

Is there a spin version of this so I can try to interpret the code better?

Mike Green said...
Set up the potentiometers as voltage dividers and use the ADC technique shown in the AN001 application note and its associated sample program.

www.parallax.com/tabid/832/Default.aspx

Mike Green · 2010-07-22 22:17

The assembly version is much faster and more accurate. Here's my hand conversion of the assembly routine to Spin:

var long sample

pub ADC(adcPinNo, fbPinNo, bits) | waitTime, cycles, oldSample
' adcPinNo and fbPinNo are the I/O pin numbers used for the analog
' to digital conversion as discussed in the comments in ADC.spin.  bits
' is the number of bits to be used for the conversion.  Note that the
' minimum number of bits in Spin would probably be 8.

' Note that, since Spin is much slower than assembly, the minimum
' sample time is much greater in Spin than in the assembly code.

   oldSample~   ' Initialize variables and registers
   cycles := |< bits - 1
   dira := |< fbPinNo
   ctra := adcPinNo | (fbPinNo << 9) | (%01001_000 << 23)
   frqa := 1
   waitTime := cnt   ' must come just before the repeat for best initial accuracy
   repeat
      waitcnt(waitTime += cycles)   ' wait for "cycles" system clocks
      sample := phsa - oldSample   ' get count since last sample time
      oldSample += sample   ' accumulate count to include this sample time

turbosupra · 2010-07-27 18:18

In reading up on ADC's, I found this object as well
http://obex.parallax.com/objects/488/
and I'm now trying to figure out my best course of action.

I'd like to control (2) 2.5 digit 7 segment LED's with the least amount of prop pins being used as possible with a BCD

Am I best to use a 4 pin BCD to give me 16 combinations, or (2) 4 pin BCD's? Maybe I could get away with (1) 5 pin BCD to drive both of them with the 32 binary posibilities?

If I understand the 4 pin LED driver BCD, it does the following, it takes 2^4 different binary combinations as an input and each combination will represent a different output that corresponds to a number on the 7 segment LED, is my understanding correct?

I'd also like to have the ability for analog values as inputs, someone asked if the prop uses SPI or IC2 as an interface, and I wasn't sure what to say. Are there certain ADC's that are better compatible with the propeller than others, a millisecond is more than enough speed for me and 2^4 is enough resolution.

Post Edited (turbosupra) : 7/28/2010 6:34:00 PM GMT

ErNa · 2010-07-28 10:09

turbosupra said...
Is this it?

http://forums.parallax.com/forums/default.aspx?f=25&m=261327&g=269023#m269023

There is a later version I will try to find it again

There is another link: http://forums.parallax.com/showthread.php?p=904925

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cmapspublic3.ihmc.us:80/servlet/SBReadResourceServlet?rid=1181572927203_421963583_5511&partName=htmltext
Hello Rest Of The World
Hello Debris
Install a propeller and blow them away

Post Edited (ErNa) : 7/28/2010 12:24:39 PM GMT

turbosupra · 2010-07-28 18:38

Thanks Erna, I will try and wrap my head around that MUX code

ErNa · 2010-07-28 19:54

The principle is: voltage measured is proportional to the counted value in relation to the elapsed time. One problem arises from the fact, that the comparator threshold is not 1/2 vdd, but in the range of 1.2 V. Therefor, zero voltage results in a non zero count, the advantage is: one can measure voltages up to -1V. Therefor you see an offset. It is tricky, but works very well. Even it the runtime changes and is never exact, calculating counts/time eliminates such fluctuations. The sampling rate must not be to long to prevent the system from overflowing 32 bits after the shift-operation.

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Hello Rest Of The World
Hello Debris
Install a propeller and blow them away

Clock Loop · 2010-07-28 21:26

I read 12 potentiometers in my black box using spin.
http://forums.parallax.com/showthread.php?p=831833

    outa[noparse][[/noparse]PotentiometerPIN] := RCState                       'set pin high or low
    dira[noparse][[/noparse]PotentiometerPIN] := 1                             'make I/O an output, charge  
    waitcnt(5_000 + cnt)                       'Wait for 1 ms to charge  
    
    dira[noparse][[/noparse]PotentiometerPIN] := 0                             'make I/O an input, discharge
    ClockCycles := cnt                         'grab clock tick counter value
    WAITPEQ(1-RCState,|< PotentiometerPIN,0)                'wait until pin goes into the opposite state you wish to measure; State: 1=discharge 0=charge
    
    ClockCycles := cnt - ClockCycles - 30000    'see how many clock cycles passed until desired State changed
    ClockCycles #>= 0                          'no less than 0

    'change the amount you divide by to match your potentiometer.(this should be the only thing you need to adjust, you might need to change the charge time also)
    Waveform := ClockCycles / 55000           

    'This code makes sure the value being stored into the FX data is within HSS limits.
    '0 to 254 
    If byte[noparse][[/noparse]@fx][noparse][[/noparse]0] <> Waveform        
      If Waveform < 1
        byte[noparse][[/noparse]@fx][noparse][[/noparse]0] := 0 
      ElseIf Waveform > 254
        byte[noparse][[/noparse]@fx][noparse][[/noparse]0] := 254 
      Else
        byte[noparse][[/noparse]@fx][noparse][[/noparse]0]:= Waveform

How to interface a rotary potentiometer with the propeller?

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