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'Best way to read a POT without the A/D?'
1998\01\07@114504 by Norm Cramer

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What is the best way to read a POT without using an A/D converter?  I need
to generate values between 0 and 128 from the reading.  It also needs to be
repeatable i.e. when the POT is not moved, the same number is generated.
The time limit for reading is not too critical, maybe 250 msec or less.
Idealy it should not need "calibration" so that multiple units would behave
the same.

What I am currently doing is the method shown at:
www.geocities.com/SiliconValley/2499/answers.html#PIC00065
and measuring the charge time.  The times seem to vary quite a bit when the
pot is in the same position.

Is there a better way to get more stable readings?

BTW I am using a 16C84 for the project.  If I can't get it to work right,
I'll switch to the 16C71 and use the A/D converter.

Thanks for any help

Norm

1998\01\07@122005 by John Payson

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> What is the best way to read a POT without using an A/D converter?  I need
> to generate values between 0 and 128 from the reading.  It also needs to be
> repeatable i.e. when the POT is not moved, the same number is generated.
> The time limit for reading is not too critical, maybe 250 msec or less.
> Idealy it should not need "calibration" so that multiple units would behave
> the same.

The charge-based RC method can work okay, but there are some caveats to be
aware of:

[1] If the capacitor is not discharged thoroughly before each reading, the
   latent charge will cause errors.

[2] If VDD is not stable, or if there are pins switching near the RC circuit
   these factors will cause errors.

[3] The pot value should be relatively small, but should have a resistor
   in series to ensure that the resistance is never zero.  The cap value
   should be moderately big to compensate for the small R, and the leads
   between the PIC and the pot should be as short as practical.

[4] Software hysteresis can work wonders at cleaning up noisy pot signals.

[5] If noise is still a problem, voltage-based pot-measurement schemes are
   much more reliable and repeatable than RC-based ones.  You may either
   use a 16C71, or you may use one of several tricks with a 16C622 or else
   a normal PIC plus external comparator.

1998\01\07@122222 by Michael S. Hagberg

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maybe you should look at an encoder. they require two inputs but are
digital.
http://www.digikey.com lists them starting at about $2.50USD.

michael

1998\01\07@130434 by Walter Banks

picon face
----------
> From: Norm Cramer <spam_OUTcramerTakeThisOuTspamDSEG.TI.COM>
> To: .....PICLISTKILLspamspam@spam@MITVMA.MIT.EDU
> Subject: Best way to read a POT without the A/D?
> Date: Wednesday, January 07, 1998 11:42 AM
>
> What is the best way to read a POT without using an A/D converter?  I
need
> to generate values between 0 and 128 from the reading.  It also needs to
be
> repeatable i.e. when the POT is not moved, the same number is generated.
> The time limit for reading is not too critical, maybe 250 msec or less.
> Idealy it should not need "calibration" so that multiple units would
behave
> the same.
>
> What I am currently doing is the method shown at:
> www.geocities.com/SiliconValley/2499/answers.html#PIC00065
> and measuring the charge time.  The times seem to vary quite a bit when
the
> pot is in the same position.
>
> Is there a better way to get more stable readings?


          --------------|   Output Bit
         |                 |
        /  Rh            |
        \                  |
         |--------------|   Vin Sense bit
        /         |        |
        \  Rl === C   |
         |        |        |
          -------
             |
           GND


   Vin the sense bit is at the threshold of the gate going into the
   micro controller. Rh and Rl are the two resistors parts of the POT
   The output bit is PWM by software to act like a variable voltage
   for the resistor chain.
                Rl                  Th
  Vin = -------------- *  ------------ * K
            Rl  + Rh         Th  + Tl

  "Th"  is the high time of the output bit and "Tl" is the low time of the

  output bit. The software to measure the pot position toggles the
  output bit to keep the Vin at the gate threshold. Solving the above
  equation will show that there is a direct relationship between the
  software PWM times and the pot position.

  Practical implementations will have two fixed value resistors on
  each end of the POT so the POT will operate over its entire range.
  The C value is non critical. It is only used for filtering.


Walter Bankls

1998\01\07@130640 by Mike Keitz

picon face
On Wed, 7 Jan 1998 11:09:37 -0600 John Payson <supercatspamKILLspamMCS.NET> writes:
> The cap
>value
>    should be moderately big to compensate for the small R, and the
>leads
>    between the PIC and the pot should be as short as practical.

Beware of "Z5U" ceramic capacitors (common in "moderately big" sizes such
as 0.1 uF).  Their capacitance changes drastically over temperature.
Either use a method that compensates for not knowing the absoulte
capacitance, or use another type of capacitor.  Capacitors with stable
capacitance are expensive in larger sizes (above 0.01 uF).

1998\01\07@141908 by DREITEK

picon face
In a message dated 98-01-07 11:48:01 EST, you write:

<<
What is the best way to read a POT without using an A/D converter?  I need
to generate values between 0 and 128 from the reading.  It also needs to be
repeatable i.e. when the POT is not moved, the same number is generated.
The time limit for reading is not too critical, maybe 250 msec or less.
Idealy it should not need "calibration" so that multiple units would behave
the same.

What I am currently doing is the method shown at:
www.geocities.com/SiliconValley/2499/answers.html#PIC00065
and measuring the charge time.  The times seem to vary quite a bit when the
pot is in the same position.

Is there a better way to get more stable readings?

BTW I am using a 16C84 for the project.  If I can't get it to work right,
I'll switch to the 16C71 and use the A/D converter.

Thanks for any help

Norm >>

Hello Norm,
I haven't looked at the URL you mentioned but I have used this method quite a
bit and it works very well.  You need to select good quality caps to prevent
drift.  Use a mylar cap.  Also experiment with the cap values.  I used a 16
bit counter instead of the usuall 8.  This steadies things up a bit.  Then you
only use the MSB of the counter for your value.  All of the drift will show up
in the LSB which you ignore.
I use this method when I need simple numeric input.  End users understand
knobs.  I made a bottled water pump for refridgerator ice makers.  The timer
on the pump was set with a simple knob.  It works great.

Dave Duley

1998\01\07@145808 by Andrew Mayo

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In essence, this approach is a single-slope integrator. Because the
comparator (the PIC port input, which ought to be a schmitt trigger
input, for maximum noise rejection) always trips on a positive-going
ramp, there is potential for some jitter due to noise. Capacitors also
suffer from various non-linearities which can affect the reading.

More sophisticated A/D approaches use dual-slope integration. In this
approach, the capacitor is first charged until the port reads a logic 1.
It is then full-charged to the supply voltage and then discharged until
the comparator reads 0. The two times are averaged.



{Quote hidden}

1998\01\07@152140 by Tom Rogers

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Well, yeah, except it does affect the swing, and therefore the noise margin;
you would like the whole range (with the added end R's, and using a given
modulation scheme) to not be too tightly centered at the threshold. So you
should at least approximate the whole waveform, i.e., do the math for the
extremes just to be sure that everything has good resolution.

I personally had good results with a differential scheme, where the cap is
charged with a known resistance and then charged with the unknown. This gets
around the tempco problems for all but the R's. Also allows the use of
"sloppy" digital inputs (as long as the stimulus conditions are the same for
both the known & unknown cycles) and can therefore be used on the simplest
PICs (without comparators, or A/Ds, or even a Schmidt trigger input)

--Tom Rogers

{Original Message removed}

1998\01\07@154602 by Norm Cramer

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At 12:25 PM 1/7/98 -0500, you wrote:
>----------
Walter,

I like the approach.  Is there any code examples that do this?

{Quote hidden}

1998\01\07@154612 by Norm Cramer

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Thought of this but I need to know the position of the pot.  It needs to
physically stop at the high limit and physically stop at the low limit.  I
couldn't find any encoders that would do this and give me 128 steps.

At 11:25 AM 1/7/98 -0600, you wrote:
>maybe you should look at an encoder. they require two inputs but are
>digital.
>http://www.digikey.com lists them starting at about $2.50USD.
>
>michael
>
>

1998\01\07@185324 by RB

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Norm

Take a look at
http://ourworld.compuserve.com/homepages/steve_lawther/bourns.htm

Bourns has a ACE (absolute contacting encoder) that provides 128 angular
postions.

                                                               Ron B

Norm Cramer wrote:

{Quote hidden}

1998\01\07@225219 by Rick Dickinson

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At 02:37 PM 1/7/98 -0600, Norm Cramer wrote:
>Thought of this but I need to know the position of the pot.  It needs to
>physically stop at the high limit and physically stop at the low limit.  I
>couldn't find any encoders that would do this and give me 128 steps.
>
>At 11:25 AM 1/7/98 -0600, you wrote:
>>maybe you should look at an encoder. they require two inputs but are
>>digital.
>>http://www.digikey.com lists them starting at about $2.50USD.

Here's an off-the-wall idea:

Use an encoder, such as the Digi-key #3315Y-1-006-ND, which gives 6 pulses
per revolution on each of the two outputs.  Thus, each revolution gives 24
detectable states.  Use two gears, one on the shaft of your knob, and one
on the shaft of the encoder, to multiply this 24 out as far as you need it.
A six-to-one ratio will give you 144 states per revolution, which will
allow you to have 128 states with only eight ninths of one knob revolution.
Physical stops at each end of the range can be achieved by adding a pin
sticking out of the knob's shaft or the side of the gear, and two
appropriately placed stops for it to bang up against.

What is the requirement that the end stops be physical?  Appropriate visual
or audible feedback of the current setting might be simple to achieve, even
with an encoder that requires multiple turns to cover the full range of
settings, and might make an appropriate substitute for "physical" feedback
(e.g.: end stops).

Hope these ideas help you think in directions that help you solve your
problem!

- Rick

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1998\01\07@232959 by Andrew Mayo

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Talking of off-the-wall ideas; this one worked experimentally though it
is quite weird.

I love those rotary encoders that have a detent action, you know, you
turn them in small clicks. Usually these consist of a cogged wheel with
a small steel strip to provide the detent action, and an optical encoder
to generate quadrature phase signals.

The other day I picked up a small permanent-magnet stepper motor (about
1 inch diameter) and twiddled the shaft idly, when suddenly the thought
occurred, what sort of transducer does a stepper motor make?.

Hooking it up to the scope, the answer was, a very good one. The motor I
used had four coils, sharing a single common ground line. Picking two
coil phases, I placed a 100 ohm resistor from each coil output to
common, then connected the outputs via two 4.7K resistors and a silicon
diode (1N914), anode to the coil,  to the bases of two NPN transistors
which had 2.2K collector resistors to +5V.

This produced an excellent quadrature square wave.

You could build, using this, some interesting gadgets. Being virtually
indestructible - after all, its a rotating armature and a set of coils -
no brushes etc, it would be quite reliable in a harsh environment e.g
dust and dirt, where optical sensors tend to fail. Fed into a PIC, which
generated a multi-phase output, you could connect an identical motor at
the output and you have a rotary positioner where the output tracks the
input (antenna rotator, for instance). Because the two motors are
identical, rotating one by x degrees will cause the other one to track
exactly. Of course, on power up the position is unknown, since it is an
open-loop system, but an index sensor would allow you to calibrate at
power up, much as a floppy disk drive does a track zero seek, looking
for the track zero photosensor to go high.

{Quote hidden}

1998\01\08@004608 by Lauw Lim Un Tung

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hi Mr.Cramer,

try astable multivibrator with IC 555, you count the frequency output
with PIC and make average with the counting for a few period.


On Wed, 7 Jan 1998, Norm Cramer wrote:

{Quote hidden}

1998\01\08@015159 by John Payson

picon face
> At 02:37 PM 1/7/98 -0600, Norm Cramer wrote:
> >Thought of this but I need to know the position of the pot.  It needs to
> >physically stop at the high limit and physically stop at the low limit.  I
> >couldn't find any encoders that would do this and give me 128 steps.
>
> What is the requirement that the end stops be physical?  Appropriate visual
> or audible feedback of the current setting might be simple to achieve, even
> with an encoder that requires multiple turns to cover the full range of
> settings, and might make an appropriate substitute for "physical" feedback
> (e.g.: end stops).

Someone wrote on here awhile ago about some encoders made by Bournes that
don't have end-stops, but output one of 128 positions directly on eight
data wires (the 8-bit output is a really wierd sort of gray code with 128
valid output values).  Perhaps something like that would be suitable?

1998\01\08@051922 by Steve Lawther

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    As Ron B has already noted, see my web page:-
   
    http://ourworld.compuserve.com/homepages/steve_lawther/bourns.htm
   
    It's not a perfect solution:-
   
    a) The encoder is 8 bit O/P (a bit greedy on I/O, unless you use a
    HC165 to serialize it)
   
    b) It has no built in mechanical stop (ie continuous rotation) - I'll
    have to look at the moulding tonight to see if bourns uses a standard
    switch package (which has holes to drop in rotation stops) or you
    could add a mechanical stop of the shaft. Both will loose you some of
    the steps as it is 128 steps in 360 deg.
   
               Steve Lawther


______________________________ Reply Separator _________________________________
Subject:      Re: Best way to read a POT without the A/D?
Author:  PC:spamBeGonesupercatspamBeGonespamMCS.NET at INTERNET-HUSKY
Date:    08/01/98 08:24


{Quote hidden}

Someone wrote on here awhile ago about some encoders made by Bournes that
don't have end-stops, but output one of 128 positions directly on eight
data wires (the 8-bit output is a really wierd sort of gray code with 128
valid output values).  Perhaps something like that would be suitable?
   

1998\01\08@075233 by Keith Howell
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Just what kind of encoder are we talking about here?

Do you really need a 7-bit absolute position encoder?
A 2-bit incremental one would use less pins.
And be cheaper.

What's the application?
Will it be turning rapidly or not?

I've been given a PIC-based hi-fi front panel to program.
It uses a 2-bit encoder to control volume and balance.

They are not as simple as they look.
They don't have the nice clean squarewaves shown in textbooks.
Maybe the optical ones do, but the mechanical ones have contact
bounce.
Cost dictates the use of mechanical encoders.
De-bouncing by software is not as easy as pushbuttons.
The data sheet says the bounce time is about 1 to 3 ms.
There are 96 phases per revolution.
If you rotate it at the maximum specified by the data sheet
(say one turn per second), each phase lasts 1/96 seconds
i.e. 10.4 milliseconds. So you need to detect valid states
that can last as short as twice the bounce time!
With hindsight, I reckon the proper thing to do is have an
RC filter (time constant around 3 milliseconds) followed by
two Schmitt triggers to sharpen the edges for the PIC.
Many PIC inputs are Schmitts, so they may be able to cope.

It's too late to change the circuit now, so I'm having to just
sample them every millisecond or so and do a bit of digital
filtering. If the same state appears 3 or 4 times, its
assumed to be valid.

Also note that if you have "clicky" ones, as I do, they may
reduce the number of settings per turn. For example, the ones
I have have one click per four phases (24 clicks per turn).
The rotor naturally settles into the clicked phase.
So if you have an electronic attenuator with 127 steps,
it takes nearly five turns to ramp through the lot, instead of
one and a third.

IMHO, clicky ones are appropriate for controlling parameters
that have discrete steps (like TV or audio channel selection).
Smooth ones are appropriate for controlling parameters
that have don't (like volume, balance etc.)

1998\01\08@090648 by Norm Cramer

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OK a few more details.

At 12:45 PM 1/8/98 +0000, you wrote:
>Just what kind of encoder are we talking about here?
>

It is a speed control for a motor (more or less).  The knob indicates the
speed.  The motor can have one of 128 speed steps sent (fine speed control
is important in the application).

>Do you really need a 7-bit absolute position encoder?
>A 2-bit incremental one would use less pins.
>And be cheaper.
>

The position of the encoder is important because when the system starts up,
it needs to set the speed to the speed the user selected on the knob.  The
user may adjust the knob while the system is off and would expect it to be
effective next time power is applied.

>What's the application?
>Will it be turning rapidly or not?
>

I doubt that it will be turning rapidly.  The max rate would be the fastest
a human could turn it.

Final cost is important also.  The production quantity may be large
relative to other components in the system.  i.e. each base unit can
support up to 16 speed controls.

1998\01\08@093325 by Andy Kunz

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Gee, Norm, my TRAIN of thought could hardly CONTROL what you might be
working on... <G>

At 07:59 AM 1/8/98 -0600, you wrote:
{Quote hidden}

==================================================================
Andy Kunz - Statistical Research, Inc. - Westfield, New Jersey USA
==================================================================

1998\01\08@105908 by Keith Howell

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Norm Cramer wrote that he was building what sounds like a control panel
for up to 16 motors. Model trains someone suggested.

> The knob indicates the speed.
> The position of the encoder is important because when the system starts up,
> it needs to set the speed to the speed the user selected on the knob.
> The user may adjust the knob while the system is off and would expect it to be
> effective next time power is applied.

> The max rate would be the fastest a human could turn it.

Well, that's fairly rapid. It's quite possible to flick a mechanical
encoder so fast that the contacts spend all their time bouncing!
Which is why optical encoding is used in things that move
reasonably fast - like mice balls.
>
> Final cost is important also.  The production quantity may be large
> relative to other components in the system.  i.e. each base unit can
> support up to 16 speed controls.

How about using the one encoder to control several items.
It's quite practical. I have a few other buttons and a
vacuum flourescent display. The first line shows the parameter name,
(volume, balance, etc) the second shows the parameter (as a bargraph,
dotgraph, numbers, text, etc). All these parameters are stored in
I2C EEPROM. So when you power up again, the settings have not changed.

For example, in a model train set controller, you could have a single
extra pushbutton. When not pressed, the rotor varies the speed of a
particular train. When the button is held down, the rotor selects which
train speed is shown on the display.

It's quite a neat concept. Consumer electronics these days have
more and more things to adjust, and this approach keeps the front panel
neat and simple.

Of course you might want two knobs if you were wanting the drama of
getting two model trains to collide!

1998\01\08@110224 by Rick Dickinson

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At 07:59 AM 1/8/98 -0600, Norm Cramer wrote:
>OK a few more details.
>At 12:45 PM 1/8/98 +0000, you wrote:

>>Just what kind of encoder are we talking about here?
>It is a speed control for a motor (more or less).  The knob indicates the
>speed.  The motor can have one of 128 speed steps sent (fine speed control
>is important in the application).

>>Do you really need a 7-bit absolute position encoder?
>>A 2-bit incremental one would use less pins.
>>And be cheaper.
>The position of the encoder is important because when the system starts up,
>it needs to set the speed to the speed the user selected on the knob.  The
>user may adjust the knob while the system is off and would expect it to be
>effective next time power is applied.

Okay, then we do need some sort of absolute position encoder.  One slightly
silly method would be to use the stepper motor-as-encoder trick proposed by
some other poster to this thread, and, upon power-up, step it down to and
end stop switch and back to see where it was set while power was off....

>>What's the application?
>>Will it be turning rapidly or not?
>I doubt that it will be turning rapidly.  The max rate would be the fastest
>a human could turn it.

>Final cost is important also.  The production quantity may be large
>relative to other components in the system.  i.e. each base unit can
>support up to 16 speed controls.

Sounds like the use of a pot as encoder is nearly inevitable, to give you
the combination of low cost per unit, and power-off-settability.

May I suggest the use of a single National ADC0819 A-D converter in the
base unit?  Specs found on the web at:

http://www.national.com/search/search.cgi/design?keywords=adc0819

These serial A/D units feature 8-bit resolution (discard the LSB for 128
states), 19 Analog inputs (enough for your 16 controls plus a few more),
and serial I/O (low pin count for interfacing to your PIC), and cost as
little as $12.04 in single-chip quantities from Digi-Key (ADC0819CCV-ND)
(that's about $0.75 for each of your 16 controllers).

- Rick "Will spec parts for food" Dickinson

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1998\01\08@120911 by DREITEK

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In a message dated 98-01-07 15:47:28 EST, you write:

<<
Thought of this but I need to know the position of the pot.  It needs to
physically stop at the high limit and physically stop at the low limit.  I
couldn't find any encoders that would do this and give me 128 steps. >>

In defence of the encoder route.  You could scale any encoder in software.
Use a common 1K/rev encoder and simply do a few shifts to scale it count.  Use
a 10 bit counter to track the encoder and simply ignore the lower 3 bits.
simple!  That would give you acurate 0 - 127 for a full rotatioin.

Dave Duley

1998\01\08@123209 by Norm Cramer

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At 03:38 PM 1/8/98 +0000, you wrote:
>Norm Cramer wrote that he was building what sounds like a control panel
>for up to 16 motors. Model trains someone suggested.
>


Sorry, not 16 controls on one panel.  up to 16 remote panels with a single
knob each.  Each remote can select which motor he wants to control and it's
speed as well as a few other things.  Each remote panel will have a PIC in
it.

The system has one base unit, up to 16 remotes that communicate with the
base unit.  Yes it is for train control but the problem can be applied to
many situations.

Thanks for all the good ideas.  Even if I don't use them all here, they are
definitly going into my archive for latter use.

Norm

1998\01\08@123219 by lilel

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> In a message dated 98-01-07 11:48:01 EST, you write:
>
> <<
>  What is the best way to read a POT without using an A/D converter?

In my business we can't afford expensive caps or $2.50 rotary
encoiders.  My whole board must cost $2.50.  I've had to substitute
smart software for expensive components.


Most responses to this message have ignored software methods of
buying noise immunity.  You have 250 Ms which is a long time to
measure your potentiometer.  This give you a chance to make several
measurements.

I've experimented with averaging, wieghted filters, and mode filters.
So far the mode filter works really well.  You take, say, 5 or 10
measurements, and sort them into a stack in RAM.  Once they are
sorted, you take the middle one.  Then you flush your RAM
stack and start over.  There's a long thread that I have a copy of
that explains all this if you are interested.


Best Regards,

Lawrence Lile

1998\01\09@014240 by Bill (WL) Boulton

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Just to ad more noise to the discussion, here's a solution that worked for
me on something similar.

Needed to input position settings via a small panel:

Used a small (cheap) rotary encoder (mechanical contact - no detents) made
by Bourne. Interfaced that to a Schmidt trigger mostly to get drive for a
one meter cable and have experienced no bounce problems. The switch does 6
cycles per revolution (24 edges) so if the contact bounce is as much as 5ms
(doubtful) you won't experience any problems unless you turn the knob at >
8 revs/sec. (I don't think so!) These encoders are stiff enough that they
don't drift when set.

NB: The only problem with contacting types is finite service life. The
problem with optical types is MONEY.

From the discussion so far, I get the impression you must be using a marked
bezzel to indicate position. Have you got the [pins, space,funds] for a 4x7
seg display. I found a decoder/display in Farnell's catalogue that uses a
serial interface over (min) 3 wires. The display is driven by the controller.

My application has range limits for input values as yours seems to, so I
simply ignore pulses which go to far. Your software should be doing that
anyway. The code I wrote dor decoding the 2 bit Gray code returns values
representing +1, -1, NoChange and WOOPS. The inputs use the "Interrupt on
Change" facility of a 16C84 and from memory the subroutine uses 2 static
and 2 dynamic bits in data memory and executes in about 23 clocks if memory
serves. Works for me...

Did I have 2 cents worth?
BILL


At 11:29 AM 8/01/98 -0600, you wrote:
{Quote hidden}

1998\01\15@013324 by Ross McKenzie

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Hmmm. Sounds like the amusement park remote control boat system that I
designed in the late '70s which used 4000 series CMOS. 16 boats each with 2
motors. Used two switches on the customer's console; one for forward/reverse
and the other for left/right (port/starboard). Centre off position =
stopped. Real fun when the park owner wanted more dramatic turns and I gave
him one motor full forward and the other full reverse!!!

Memory lane walking ......

Ross McKenzie
Melbourne Australia


At 11:29 AM 1/8/98 -0600, you wrote:
{Quote hidden}

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