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'[EE]: How to convert Resistance to digital data di'
2001\04\29@124753 by Bob Blick

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>I want to use a microcontroller to monitor and measure the change in
>resistance value of a variable resistor.

Any particular microcontroller? How about a PIC?

>But I want to do the resistance to
>digital data conversion directly without using an ADC in order to reduce
>chip counts and cost. Accuracy is not significant in my requirement. Is it
>possible?

Yes with a PIC you can use a capacitor and a resistor and one PIC pin, plus
your unknown resistor. Don't know why I am the first to suggest it this
way, it is the simplest tried-and-true method.

Use any PIC pin on port B or any pin except RA4 on port A.

220 ohm resistor from the PIC to the capacitor. Other end of the cap to
ground. Attach your mystery resistor from the cap to + supply voltage.

Make the pin an output and make it low for a good long time, say 1 to 10
milliseconds, so you know the cap is as close to ground as it's going to get.

Then make the pin an input and time how long it takes to change from a 0 to
a 1. Longer time = higher resistance.

Rinse, and repeat as neccessary.

The cap value depends on your resistor and your timing loop. This method
does not work for very small resistor values(the 220 ohms protects the PIC
pin from a capacitive load but also limits the lowest resistance you can
measure.

Repeatability is about +-2 percent.

Cheerful regards,

Bob

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2001\04\29@132345 by David VanHorn

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>
>Repeatability is about +-2 percent.

Not from where I sit..

I'm working on this, and not having much luck with it.
I tried 1-15k resistors, and got horrible non-linearity below about 10k.
Now using 10-150k, I'm getting jitter on the low threshold that's munging
the time by a factor of 2 or more.

Tried 0.1uF, and 0.01uF, no particular difference other than scale.

I'm using the MPLAB-ICE, and four channel DSO to diagnose. (Yes I factored
in the probe resistance).

My biggest frustration is that the discharge dosen't seem to be stable to
more than about 0.5V, and sometimes dosen't detect till as near to zero as
I can see.

I start the discharge, start timer 1, then sit in a loop of
Here:
  BTFSC PORTB,4
  goto Here

No ints, nothing else going on..

I agree that it SHOULD work, and I was rather counting on it to work, but
it dosen't seem to BE working.

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2001\04\29@133846 by Bob Blick

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At 12:09 PM 4/29/2001 -0500, you wrote:
>>
>>Repeatability is about +-2 percent.
>
>Not from where I sit..
>I'm using the MPLAB-ICE,

That's not gonna help. 0 threshold is pretty consistent on a plain PIC.

Also, repeatability is +-2 percent, not linearity. Averaging the values I
was able to get +-1 degree repeatability in a thermostat this way(using a
thermistor). This was on several units, I don't think it's a fluke.

Cheers,

Bob

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2001\04\29@135311 by David VanHorn

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At 10:36 AM 4/29/01 -0700, Bob Blick wrote:
>At 12:09 PM 4/29/2001 -0500, you wrote:
> >>
> >>Repeatability is about +-2 percent.
> >
> >Not from where I sit..
> >I'm using the MPLAB-ICE,
>
>That's not gonna help. 0 threshold is pretty consistent on a plain PIC.
>
>Also, repeatability is +-2 percent, not linearity. Averaging the values I
>was able to get +-1 degree repeatability in a thermostat this way(using a
>thermistor). This was on several units, I don't think it's a fluke.

What am I missing then?
I use the method you described, charging the cap for several RC periods
till it's pretty much flat.
Then I enter the discharge timing loop, and all I can do is exit when the
pic tells me the pin is low.
The loop is two instructions at 1 MHz, and I'm getting timing values in the
range of 0AXh for 10k, so I've got plenty of resolution, but I see it in
the data, and on the scope, that the discharge termination point (and
presumably the low threshold) is bouncing around like crazy.

I'm not so worried about linearity, at 10k that seems to not be a problem,
but I can't deal with huge variability between measurements when nothing
(intentionally) is changing.

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2001\04\29@141639 by Bob Blick

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>I use the method you described, charging the cap for several RC periods
>till it's pretty much flat.
>Then I enter the discharge timing loop, and all I can do is exit when the
>pic tells me the pin is low.
>The loop is two instructions at 1 MHz, and I'm getting timing values in the
>range of 0AXh for 10k, so I've got plenty of resolution, but I see it in
>the data, and on the scope, that the discharge termination point (and
>presumably the low threshold) is bouncing around like crazy.

Only difference is I used it the opposite way around, measuring the time it
takes to charge. Your method should be more accurate.

Actually I counted T0 interrupts, so my loops were very long and I used a
bigger cap, but that should make my results even worse than yours.

Don't know why it's not working for you, but you're smart so I imagine
you'll squash the gremlin.

Cheers,

Bob

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2001\04\29@142651 by David VanHorn

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>Don't know why it's not working for you, but you're smart so I imagine
>you'll squash the gremlin.

I've got my BFH ready for that event.


It appears that the low threshold voltage is jittering, but sometimes it's
practically at ZERO, and that makes no sense at all.  I can't believe that
my little 2 instruction loop is introducing any real latency.
A couple of counts, yes, but that's why I am using a 16 bit measurement on
the discharge time, and a large-ish 8 bit "1" value.

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2001\04\29@161615 by Russell McMahon

picon face
What sort of caps are you using?
Presumably Mylar or similar rather than eg ceramic.

Does transferring this from the ICE to a "real" PIC produce an identical
result?

Presumably when you say "non-linearity" you mean non linear compared to an
exponential curve.

Might it help to eg go low, then high, and time for say 5 time constants
after the low to high transition and then go low again and count the time
taken? That way, even though the 5 time constants "should" pretty much have
the cap at top rail, driving it there for a consistent period after it
passes through low-high threshold may reduce variability.




     Russell McMahon

{Original Message removed}

2001\04\29@162621 by David VanHorn

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At 08:16 AM 4/30/01 +1200, Russell McMahon wrote:
>  What sort of caps are you using?
>Presumably Mylar or similar rather than eg ceramic.
>
>Does transferring this from the ICE to a "real" PIC produce an identical
>result?

Difficult. I've only got one board, and it's SMD.

>Presumably when you say "non-linearity" you mean non linear compared to an
>exponential curve.
>
>Might it help to eg go low, then high, and time for say 5 time constants
>after the low to high transition and then go low again and count the time
>taken?

That's what I'm doing.  Charge for long time (verified on scope to flat)
then discharge and time.


>That way, even though the 5 time constants "should" pretty much have
>the cap at top rail, driving it there for a consistent period after it
>passes through low-high threshold may reduce variability.

I'm charging it way into the flat.
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2001\04\29@163030 by David VanHorn

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At 08:16 AM 4/30/01 +1200, Russell McMahon wrote:
>  What sort of caps are you using?
>Presumably Mylar or similar rather than eg ceramic.

Ceramic at the moment.
I discharge through a fixed R, then through two variable Rs (in turn) so my
answer should be:

S1=V1/Fixed and S2 = V2/Fixed  With S1,2 = 0-15.

This way, the absolute value of the cap isn't so important, and drift isn't
a factor.
There is also a routine, during the establishment of the Fixed value, that
steps down the T1 prescaler, looking for the largest result that will fit
in an 8 bit word (T1H= 0)

That way, the routine is maximally tolerant of absolute values.

My biggest problem is this apparent variability in the low threshold, for
reasons that are not obvious.


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2001\04\30@001138 by Scott Dattalo

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On Mon, 30 Apr 2001, Russell McMahon wrote:

>  What sort of caps are you using?
> Presumably Mylar or similar rather than eg ceramic.
>
> Does transferring this from the ICE to a "real" PIC produce an identical
> result?
>
> Presumably when you say "non-linearity" you mean non linear compared to an
> exponential curve.
>
> Might it help to eg go low, then high, and time for say 5 time constants
> after the low to high transition and then go low again and count the time
> taken? That way, even though the 5 time constants "should" pretty much have
> the cap at top rail, driving it there for a consistent period after it
> passes through low-high threshold may reduce variability.

Good points.

I'm curious how this "2-instruction loop" is implemented. How can this be done
AND count the number of cycles? I know one way, but from the context I'm sure
that's not being done.

To achieve the +/- 2% repeatibility to which Bob alludes you'll have to be
extremely careful in keeping track of ALL cycles and ALL voltages.

First the voltages: you've got ground, Vcc, and the two switching thresholds of
the I/O pin (the low-to-high and the high-to-low).

Others here have more experience than me with the PIC I/O thresholds. But one
thing I'd comment on is that the thresholds are sensitive to variations in
Vcc. If the sensitivity is linear then this approach is ratiometric. But, I
don't know how the input thresholds vary. It'd be instructive to create a simple
program to measure the input voltage thresholds:

l1:
   bcf   ioport, bitx    ; The ioport we'll monitor
l2: btfss ioport, bity    ; the ioport/bit we'll test
    goto l1
   bsf   ioport, bitx
   goto  l2

Attach a variable power supply (or a the wiper of a pot) to the ioport you wish
test. Run this program and monitor the other I/O pin.

This will give you a good idea of the thresholds at a given vcc. In your lab
notebook, you may wish to record the results for a few different Vcc's
(e.g. 4.5, 4.75, 5.0, 5.25 etc)

-----------------

Now, the algorithm I'd take is this:

1) Charge the cap. You can either do this through your unknown resistor or
tie the cap to another I/O pin that can quickly charge the cap. The latter
makes more sense since you can use the "discharge" I/O as your input to monitor
the capacitor voltage, so I'll assume that in what follows.

Eg:

       |
PIC I/O +----+------------+
       |    |            |
       |    |            /
       |   ===  C Ref    \ R unknown
       |    |            /
       |    |            |
           ---          ---
          ///          ///


2) Begin charging the cap. Make sure it's fully charged! The pic I/O pin has
fairly low output impedance, so should charge the cap in a much shorter time
than your unknown resistor.

3) At the same time you change the output pin that was charging the cap to an
input, start a timer. Note that the I/O pin that was charging the cap is now
monitoring the cap's voltage.

4) When the PIC I/O goes low, stop the timer.

------

Sounds easy enough.

But to get repeatable results you'll need an accurate counter. I don't know
which PIC you're using. If you're using one with the Timer 1 peripheral (like
the F877) then the timer is trivial. If you're confined to the '84 then you'll
probably want to grab my 3-instruction cycle resolution software pulse width
measurer. It's part of the gpsim distribution and can be found in CVS:

http://cvs.sourceforge.net/cgi-bin/viewcvs.cgi/gpsim/examples/14bit/

the file is pulse_measure.asm


Scott

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2001\04\30@003905 by David VanHorn

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>
>Good points.
>
>I'm curious how this "2-instruction loop" is implemented. How can this be done
>AND count the number of cycles? I know one way, but from the context I'm sure
>that's not being done.

With the timer running, and the cap discharging:

Here: BTFSC     PORTB,Pin
        goto    Here


>To achieve the +/- 2% repeatibility to which Bob alludes you'll have to be
>extremely careful in keeping track of ALL cycles and ALL voltages.

What I'm referring to isn't a 2% sort of thing.
The discharge time is arbitrarily doubling due to an apparent change in the
low logic threshold.


>First the voltages: you've got ground, Vcc, and the two switching
>thresholds of
>the I/O pin (the low-to-high and the high-to-low).
>
>Others here have more experience than me with the PIC I/O thresholds. But one
>thing I'd comment on is that the thresholds are sensitive to variations in
>Vcc.

I'm aware of that, but I'm not seeing anything significant on VCC.
This is all happening on an MPLAB ICE, so I'm calling uChip tomorrow.


>This will give you a good idea of the thresholds at a given vcc. In your lab
>notebook, you may wish to record the results for a few different Vcc's
>(e.g. 4.5, 4.75, 5.0, 5.25 etc)

Should all be factored out, as long as the thresholds don't change between
measurements.
That's the problem I'm hitting.
The low threshold appears to go absurdly low, practically to zero on occasion.

{Quote hidden}

A bit more complicated than that.
I have three pic pins connected to the cap.
One has a fixed 10k ref value.
The other two have 0-150k jumperable in 10k increments.
There is no charge or discharge except through a pic pin.



>2) Begin charging the cap. Make sure it's fully charged! The pic I/O pin has
>fairly low output impedance, so should charge the cap in a much shorter time
>than your unknown resistor.

BTDT, I'm WAY into the flat part of the curve.
I'm using a pin through a 10k, which is my known component in the system.
That pin then is used to determine the discharge through the 10k, which
factors out the cap value.
Then I measure discharges through the active pins with 0-150k in 10k steps,
and divide by the time observed in the reference measurement.


>3) At the same time you change the output pin that was charging the cap to an
>input, start a timer. Note that the I/O pin that was charging the cap is now
>monitoring the cap's voltage.

I could charge through the discharge resistors, but that could take up to
15x, and I have no way (yet) to know what's jumpered in. So, I charge
through my 10k fixed R.


>4) When the PIC I/O goes low, stop the timer.

This is where the problem comes in.

>------
>
>Sounds easy enough.
>
>But to get repeatable results you'll need an accurate counter. I don't know
>which PIC you're using. If you're using one with the Timer 1 peripheral (like
>the F877) then the timer is trivial.

'876 running at 1 MHz.
There's a preset routine that starts the counter fast, and re-reads the
standard 10k, changing the prescaler till it gets an 8 bit value, assuring
that the granularity, and the odd instruction cycle difference won't be a
problem.


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2001\04\30@040309 by Michael Rigby-Jones

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I have used the single slope ADC concept to consistently measure the value
of the thermistor to within 0.1 degree C and the final result worked very
well.  I was measuring the charging time of the cap, and comparing it to a
fixed reference in order to cancel supply voltage variations.  Actually it
was slightly more involved than that, the first version compared the
thermistor to the value of a digital pot by which was used to adjust the set
point of the control loop.  This worked well.  The second version had the
set point stored in software, and the digital pot replaced by a fixed
reference.  This didn't work so well!

One thing I found was that using an ICE (both ICEPIC2 and MPLAB-ICE) totaly
screwed the readings and introduced large amounts of jitter on the results.
This wasn't so bad for my development as I was dealing with a control loop
with a large time constant (several seconds) and the jittered results tended
to average out.  Using a proper PPIC on the PCB totaly cured the jitter
problems.  It looks like the ICE introduces a fair amount of noise onto the
port lines, which under normal (i.e. digital) use would probably be
unaffected.

Regards

Mike

> {Original Message removed}

2001\04\30@051450 by Snail Instruments

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>>  What sort of caps are you using?
>>Presumably Mylar or similar rather than eg ceramic.
>
>Ceramic at the moment.

You might also experience charge absorption effect here.

>My biggest problem is this apparent variability in the low threshold, for
>reasons that are not obvious.

A Vdd or ground ripple ?

Josef


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2001\04\30@070854 by Bob Ammerman

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> I'm curious how this "2-instruction loop" is implemented. How can this be
done
> AND count the number of cycles? I know one way, but from the context I'm
sure
> that's not being done.

By using a timer to do the counting.


Bob Ammerman
RAm Systems
(contract development of high performance, high function, low-level
software)

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2001\04\30@092315 by Roman Black

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David VanHorn wrote:
{Quote hidden}

Hi David, maybe you should try a much smaller cap
for faster discharge. I remember Jinx posting about
problems with the PIC threshold levels when the
voltage changed too slowly.

Other than that maybe restructure to just one
resistor and one cap and one PIC pin and so some
testing that way. Remember the PIC pins are tied
internally as "ports" with limits for total port
current and stuff, they may have weird effects on
each other's threshold levels etc. Just fishing!
:o)
-Roman

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2001\04\30@185623 by Russell McMahon

picon face
> >  What sort of caps are you using?
> >Presumably Mylar or similar rather than eg ceramic.
>
> Ceramic at the moment.

Try a Mylar or similar.
MAY not help but don't be amazed if it does.
I am running a Sigma Delta converter with the integrating cap at abourt 1uF.
This is effectively a charge balancing converter - the cap is held more or
less at a constant voltage by a comparator and the processor feedback loop.
It varies slightly from the reference value due to the time quantisizing of
the decision which is an inherent part of the SD process. Anyway - the point
is that the voltage ripples SLIGHTLY about the reference point. A ceramic
capacitor is USELESS here - accuracy is much much worse than when a quality
"plastic" cap is used.
Try it in your case. Easy, quick, cheap and just maybe ....



Russell McMahon

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2001\04\30@190048 by Peter L. Peres

picon face
> It appears that the low threshold voltage is jittering, but sometimes
> it's practically at ZERO, and that makes no sense at all.  I can't
> believe that my little 2 instruction loop is introducing any real
> latency. A couple of counts, yes, but that's why I am using a 16 bit
> measurement on the discharge time, and a large-ish 8 bit "1" value.

Any interrupts turned on by chance ?

I use code like:

loop:
 movlw SET_OUTPUT_1
 movwf OUTPUT
 btfss INPUT.BIT
 goto  $-1

 goto  $+1     ;; compensate jump timing

 ;; optional delay here

 movlw SET_OUTPUT_0
 movwf OUTPUT
 btfsc INPUT.BIT
 goto  $-1

 ;; optional delay here

 goto  loop

to test RC circuits on 'new' configurations. The output is a scope probe
and a counter (slaved to scope X1 channel). This will not detect jitter
that averages out but it's pretty good for getting in the 'ballpark' for
the counter value(s) quickly. NOTE that I always do this with a Schmitt
input. An extra fixed period of pushing high and low may be required after
the detected transition if not. After collecting some values and heat
cycling I get a workable code based on this in 3 tries or so. A test
circuit based on this is also handy when examining parts for building a
set of devices based on such principles.

good luck,

Peter

PS: In fact I am going to use this again 2morrow as I am working on a
12C508 based project ;-)

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'[EE]: How to convert Resistance to digital data di'
2001\05\01@033656 by David Cary
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Dear David VanHorn,

David VanHorn <RemoveMEdvanhornTakeThisOuTspamCEDAR.NET> on 2001-04-29 11:25:19 PM wondered what the
problem could be:
> The discharge time is arbitrarily doubling due to an apparent change in the
> low logic threshold.
...
> This is all happening on an MPLAB ICE, so I'm calling uChip tomorrow.
...
> Should all be factored out, as long as the thresholds don't change between
> measurements.
> That's the problem I'm hitting.
> The low threshold appears to go absurdly low, practically to zero on occasion.

That does sound bizzare.

Maybe you could send the capacitor voltage to a real comparator, and watch the
output of the comparator on your o'scope. Do you see a output glitch on the
comparator on your o'scope long before the expected time ? Then drive the
charge/discharge with 1 PIC pin and read the comparator with another PIC pin.
That would help narrow down whether the problem is with the PIC (does it exit
the loop before the comparator switches ?) or with the analog hardware (see
glitches on comparator output).

If that works, you could replace the comparator with a simple (schmitt trigger)
inverter chip, and finally a straight wire directly into the PIC. You are using
a schmitt trigger input, right ?

> A bit more complicated than that.
> I have three pic pins connected to the cap.
> One has a fixed 10k ref value.
> The other two have 0-150k jumperable in 10k increments.
> There is no charge or discharge except through a pic pin.

Mmm... I'm reaching here, but maybe it's noise from the ICE being picked up in
the 10 KOhm reference resistor. (That's the one you're using as input to see how
long the capacitor charges/discharges, right ?). Maybe try a (Schmitt trigger)
PIC pin dedicated to input connected directly to the cap (zero Ohm).

10 KOhms to +5V gives at most 500 uA. That sounds suspiciously close to the 400
uA maximum PIC internal pull-up current ...

Reaching for straws even further, maybe you're somehow driving a pin into some
state such that when you switch to an input, parasitic inductance and
capacitance keep it in that state for a few microseconds, and the next read
gives a false value.

Tell us when you figure it out, OK ?

--
David Cary

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2001\05\01@040419 by Roman Black

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David Cary wrote:
>
> Dear David VanHorn,
>
> David VanHorn <TakeThisOuTdvanhornEraseMEspamspam_OUTCEDAR.NET> on 2001-04-29 11:25:19 PM wondered what the
> problem could be:
> > The discharge time is arbitrarily doubling due to an apparent change in the
> > low logic threshold.
> ...
> > This is all happening on an MPLAB ICE, so I'm calling uChip tomorrow.
> ...
> > Should all be factored out, as long as the thresholds don't change between
> > measurements.
> > That's the problem I'm hitting.
> > The low threshold appears to go absurdly low, practically to zero on occasion.


That does sound bizarre, David, have you checked the
+5v rail as *maybe* you are getting some sort of
charge-pump effect where the constant charge/discharge
of the sense cap is affecting the main Vdd that the
PIC uses to determine threshold voltages for input
pins?? Time to put the scope probe on the +5v
rail and the sense cap to see what voltages you are
really getting there. :o)
-Roman

PS. also re my previous post, try making just ONE
sense pin on it's own port. Each port has it's
own internal power rail with it's own thresholds
(obviously).

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