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'BCD Thumbwheels'
1997\08\19@120629 by Alec Myers

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Does any know a cute way of reading three BCD thumbwheels that uses less than
       i)      Twelve i/o pins
       ii)     Seven pins and 12 diodes?

1997\08\20@005352 by Andrew Warren

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Alec Myers <spam_OUTPICLISTTakeThisOuTspamMITVMA.MIT.EDU> wrote:

> Does any know a cute way of reading three BCD thumbwheels that uses
> less than
>         i)      Twelve i/o pins
>         ii)     Seven pins and 12 diodes?

   Yeah, Alec... You can do it with only three I/O pins if you're
   willing to add a little external hardware:

   Get two 8-bit serial shift-registers and cascade them to make a
   16-bit shift-register.

   Now tie 12 of the shift-register outputs to one side of each
   thumbwheel switch.  Tie all 12 pins on the other side of the
   switches together, run them to a single I/O pin on the PIC, and
   put a pull-down resistor on that pin.  Configure that pin as an
   INPUT.

   Tie the shift-register Latch (or Strobe) pins together and pull
   them high (or low if the Latch/Strobe signal is low-true).

   Tie the shift-register Clock pins together and run them to an
   I/O pin on the PIC.  Make that pin an OUTPUT.

   Tie the first shift-register's Data pin to a PIC I/O pin and
   make that pin an OUTPUT, as well.

   Now...

   To read the switches, first clear the shift-registers by
   clocking 16 "0" bits into them.  Then clock a "1" bit in and
   read the INPUT pin.  If it reads "1", the first switch is
   closed; if it reads "0", that switch is open.

   Clock 11 "0" bits into the shift register, one at a time.  This
   will push that single "1" bit through the shift-register,
   applying it to each switch in turn.  After each "0" bit is
   clocked in, check the INPUT pin; its state indicates whether the
   corresponding switch is open or closed.

   Shift registers aren't as cheap as diodes, but since you're
   already spending the bucks for those thumbwheel switches, you
   can probably afford them.

   If this didn't make sense to you, let me know and I'll try to
   clarify my description.  If you ask real nicely, I may even draw
   up some really awful ASCII art to illustrate it.

   -Andy

=== Andrew Warren - .....fastfwdKILLspamspam@spam@ix.netcom.com
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1997\08\20@012320 by Andrew Warren

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I wrote:

> Now tie 12 of the shift-register outputs to one side of each
> thumbwheel switch.  Tie all 12 pins on the other side of the
> switches together, run them to a single I/O pin on the PIC, and put
> a pull-down resistor on that pin.  Configure that pin as an INPUT.
> ....
> push [a] single "1" bit through the shift-register, applying it to
> each switch in turn.  After each [clock], check the INPUT pin; its
> state indicates whether the corresponding switch is open or closed.

   Uhh... A small, but important, correction:

   The above method will only work if there's a PIC-facing diode
   between each switch and the PIC's INPUT pin.

   If you want to dispense with the 12 diodes, you need to find a
   shift-register with open-collector outputs and invert the 1's and
   0's (that is, fill the shift-register with "1" bits and clock a
   single "0" bit through it).

   Such a shift-register DOES exist, but as I recall, it was very
   hard to find.  I'll go through my notes and see if I can find a
   part number (or if anyone else knows of a shift-register with
   open-collector outputs, please post the part number to the
   list).

   -Andy

=== Andrew Warren - .....fastfwdKILLspamspam.....ix.netcom.com
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1997\08\20@020422 by Peter Homann

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Andrew Warren wrote:
{Quote hidden}

Hi,

Maybe I've missied something, but, is it not easier (no diodes and
standard shift-register) to use the shift-register to shift IN the
switch positions.

In a normal setup the BCD outputs of the switch present a logic 1 or 0.
Therefore, if they are connected to the 12 parallel shift register
inputs, the value of each input would be serially shifted in.

Peter.
--
_______________________________________________________________________
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Adacel Pty Ltd                                   Fax  : +61 3 9596-2960
250 Bay St, Brighton 3186, VIC, AUSTRALIA      Mobile :     014 025-925

1997\08\20@031117 by Andrew Warren

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Peter Homann <@spam@PICLISTKILLspamspamMITVMA.MIT.EDU> wrote:

> Maybe I've missied something, but, is it not easier (no diodes and
> standard shift-register) to use the shift-register to shift IN the
> switch positions.
>
> In a normal setup the BCD outputs of the switch present a logic 1 or
> 0. Therefore, if they are connected to the 12 parallel shift
> register inputs, the value of each input would be serially shifted
> in.

Peter:

Good suggestion, but the switches I've seen don't "present a  logic
1 or logic 0"; they just open or close.  Therefore, you'd still need
12 pullup or pulldown resistors, one for each switch.

Nevertheless, your method's better if you can't find the
open-collector-output shift-register that I mentioned, since
resistors are cheaper than diodes.

-Andy

=== Andrew Warren - KILLspamfastfwdKILLspamspamix.netcom.com
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1997\08\20@064138 by mikesmith_oz

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On 19 Aug 97 at 15:45, Alec Myers wrote:

> Does any know a cute way of reading three BCD thumbwheels that uses
> less than
>         i)      Twelve i/o pins
>         ii)     Seven pins and 12 diodes?
>

I wouldn't really recommend it - but you said 'cute' ...

3 io pins and 12 resistors - but the io pins have to be a/d

A variant of the above would be using schmitt inputs (some of the
pics ios are) and using a capacitor - measure time.  You'd probably
need to calibrate.

Realistically, I'd go for your solution 2 - probably using the soft
pullups on port B in lieu of the resistors you'll still need and
didn't mention.
MikeS
<spamBeGonemikesmith_ozspamBeGonespamrelaymail.net>

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1997\08\20@100651 by paulb

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Alec Myers wrote:

> Does any know a cute way of reading three BCD thumbwheels that uses
> less than
>         i)      Twelve i/o pins
>         ii)     Seven pins and 12 diodes?

 Yes, use a 4000 series parallel-in, serial out shift register with the
requisite number of inputs (just looking for one in the specs!).  That
would require three PIC lines, two of which can be used for another
function as well (the Latch Enable and serial clock - the data line is
simply ignored while these are otherwise used).

 Cheers,
       Paul B.

P.S.: You will need pullup networks.

1997\08\20@122007 by Harold Hallikainen

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       Reading nonmontary switches such as thumbwheels and dip switches
is always a pain since one way or another you need lots of I/O pins
(either on the chip itself or using a parallel to serial shift register).
I've managed to get a few products done here using the 16c74a and just
have the 3 digit thumbwheel drive 12 input lines.  8 of them are on port
B with internal pull-ups, so pull-ups are only needed on 4 lines.
       I generally think eeprom is a way around using thumbwheel and dip
switches.  Like the switches, eeprom is nonvolatile and can be changed by
the user.  Unfortunately, the user can't see the current settings.  You
then end up with some sort of display to show the current settings and
prompt the user along with momentary key switches (that can be scanned)
for user input.  Where this is not cost effective, I think I'd either
drive the pic I/O lines directly, if there are enough, or go with the
external 12 bit shift register.  That shift register can be expanded to
handle more input lines, if needed.
       It's all a trade-off!

Harold

1997\08\20@131528 by Mike

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At 12:18 PM 8/20/97 EDT, you wrote:

WHy not treat the thumbwheels as if they are keys annd use diodes, selecting
the group via one of three port pins -  Ah la HC139 etc ?

Rgds

Mike
Perth, Western Australia

1997\08\20@140235 by ame

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    From:    Alec Myers <RemoveMEAlecspamTakeThisOuTSHRUBBERY.DEMON.CO.UK>

    Does any know a cute way of reading three BCD thumbwheels that uses
    less than
            i)      Twelve i/o pins
            ii)     Seven pins and 12 diodes?

    ---

    Try a latch and a shift register:  3 or maybe 4 pins.

    Tristate buffers on each thumbwheel: 2 for address, 4 for BCD value,
    and since you are using 3 switches the 4th binary state on the address
    pins allows you to reuse the 4 BCD address lines for something else.

    Nothing else springs to mind right now.

    Andy #2

1997\08\20@141843 by Mike

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At 03:43 PM 8/20/97 +1000, you wrote:

>In a normal setup the BCD outputs of the switch present a logic 1 or 0.
>Therefore, if they are connected to the 12 parallel shift register
>inputs, the value of each input would be serially shifted in.
>
>Peter.

They are normally open/close switches, the cheap ones don't have diodes
or pullups built in.

Me thinks the cheapest solution is to use a pair of 74HC165's or
equivalents and a resnet which has 13 pullups in one package or a
pair of 9pin sips since these are produced en masse. All the PIC has
to do is supply 3 I/O pins. Two output and one input - the benefit of
this approach is you have 4 bits left over since 2x165's give 16bit
input - no diodes needed and low PCB space...

Rgds

mike
Perth, Western Australia

1997\08\21@002840 by Dana Raymond

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Date:    Tue, 19 Aug 1997 15:45:42 +0100
From:    Alec Myers <AlecEraseMEspam.....SHRUBBERY.DEMON.CO.UK>
Subject: BCD Thumbwheels

> Does any know a cute way of reading three BCD thumbwheels that uses less than
>         i)      Twelve i/o pins
>        ii)     Seven pins and 12 diodes?

See if there is a 12 bit parallel-in, serial-out shift register in the
40xxx or 74HCxxx line. 3 lines can be used to read the switches: Clock,
Data, and Load. The Load line causes the shift register to read the
switches. The clock is then toggled as the data is read in. You can even
save a pin by adding a diode, resistor, and capacitor from the clock to
the load line on the shift register. These components cause the Load
line to be asserted when the Clock line is idle for a period of time.

I've used this technique sucessfully for output expansion using 2 pins,
a resistor/cap/diode network, and a 4094 serial-in, parallel-out shift
register with output load pin. I don't see any reason why it won't work
in reverse with the appropriate part. I'd find one for you, except that
I'm not near my library at the moment.

Don't forget the 12 pullup resistors.

Hope this helps.

Dana Raymond,
EraseMEdfrspamicom.ca

1997\08\21@015218 by William Chops Westfield

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Speaking of which, I'll throw out a problem for the group mind to ponder...
Given N I/O pins, what's the largest number of values that you can come up
with via create connection to some number of SPST switches (ie dipswitches
or jumpers)??  (lets say you can use passive components as well, but the
idea is to minimize size and complexity.

2^N is trivial.
I'm pretty sure you can do 3^N by detecting NC vs Gnd/VCC.
You might get 4^(N-1) by using (NC, GND, VCC, IO(N)).
You can use simple A-D conversion (ala basic stamp ap notes) in conjunction
with some resistor ladders and caps, and probably easilly get several bits
per pin, although this is more complex than I'd like to get (ie 3 bits per
pin would require 8 voltage levels to be available.)

It's not just an idle question, of course.  Given an 8pin PIC, you want
every pin to do as much as possible!

BillW

1997\08\21@024525 by Andrew Warren

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Mike <RemoveMEPICLISTEraseMEspamEraseMEMITVMA.MIT.EDU> wrote:

> WHy not treat the thumbwheels as if they are keys annd use diodes,
> selecting the group via one of three port pins -  Ah la HC139 etc ?

Mike:

That's the "seven pins and twelve diodes" method that was mentioned
in the original question.  The diodes are necessary because ALL
combinations (from no sitches closed all the way up to all 12
switches closed) need to be unambiguously read.

-Andy

=== Andrew Warren - RemoveMEfastfwdspam_OUTspamKILLspamix.netcom.com
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1997\08\21@050028 by Frank A. Vorstenbosch

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Paul B. Webster" <EraseMEpaulbspamspamspamBeGoneMIDCOAST.COM.AU> wrote:
> Subject: Re: BCD Thumbwheels
>
> Alec Myers wrote:
>
>> Does any know a cute way of reading three BCD thumbwheels that uses
>> less than
>>         i)      Twelve i/o pins
>>         ii)     Seven pins and 12 diodes?
>
>   Yes, use a 4000 series parallel-in, serial out shift register with the
> requisite number of inputs (just looking for one in the specs!).

There is such a shift register in the 7400 series.  Of course you'll need
twelve pull-up (or pull-down if CMOS) resistors as well.  From the GIICM
(http://www.xs4all.nl/~falstaff/GIICM.html) we get this pinout:

74674
16-bit parallel-in serial-out shift register.

     +---+--+---+
 /CS |1  +--+ 24| VCC
SCLK |2       23| P15
R//W |3       22| P14
     |4       21| P13
MODE |5       20| P12
S/Q15 |6   74  19| P11
  P0 |7  674  18| P10
  P1 |8       17| P9
  P2 |9       16| P8
  P3 |10      15| P7
  P4 |11      14| P6
 GND |12      13| P5
     +----------+

There is an even more peculiar way of doing things if you can't find the
74674.  Use a (74HC)4040 12-bit ripple counter, and connect diodes to
the outputs, and then from the diodes connect the switches to one common
input pin with suitable pull-down resistor.  Then reset the counter, and
clock to the states 1, 2, 4, 8, 16, 32, 64, 128, 256, 512, 1024 and 2048,
each time reading the state of the input line.  Of course, this may not
be the fastest way, requiring approx. 5 PIC clock cycles per 4040 clock,
so that is about 11000 cyles to read the twelve switches.

Frank
------------------------------------------------------------------------
Frank A. Vorstenbosch                           Phone:  +44-181-636 3391
Electronics & Software Engineer                    or:  +44-181-636 3000
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1997\08\21@073652 by Russell McMahon

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Alec Myers <RemoveMEPICLISTKILLspamspamMITVMA.MIT.EDU> wrote:

> Does any know a cute way of reading three BCD thumbwheels
that uses
> less than
>         i)      Twelve i/o pins
>         ii)     Seven pins and 12 diodes?

>Andrew Warren wrote:
>
>   Yeah, Alec... You can do it with only three I/O pins if
you're
 >  willing to add a little external hardware:
>
>    Get two 8-bit serial shift-registers and cascade them to
make a
>    16-bit shift-register.
>
>    Now tie 12 of the shift-register outputs to one side of
each

Andrew's suggestion can provide a very cheap solution if you
can tolerate the extra board area.
As an additional advantage the i/o may be physically some
distance away.
I am using an ST6* (groan) in an application where I need a
total of about 50 i/o pins. 20 of these are on the CPU proper
and the rest are provided by using 4021 shift registers for
input and 4094 shift registers for output. These are VERY cheap
and very easy to drive. Cost per IC is about $NZ0.30 in about
100 quantities - probably under $US0.20 each. Its much simpler
and more compact than IIC as there is no addressing per se -
you just clock in or out the correct number of bits. In fact I
add an extra 16 or 32 bits under certain circumstances as the
device (a talking communicator for non-verbal people) has
various optional remote add ons - I place these on the end of
the chain formed by the internal shift registersand clock the
whole chain a suitable number of times. No clearing of
registers is needed but there is a strobe (to load), clock and
data signal to contend with. In my application I common the
clock for input and output as only one direction gets used at a
time.I can drive this arrangement down a 2 metre cable with no
problems . Speed is as fast as an St6 can go at 6 MHz (which is
very slow!) - probably about 10 or 20 microseconds per bit on
the bus !!!!.
It should be possible to go mucvh faster over the same cable.

eg

INPUT:

N =  number of bits in chain.
Assert strobe                           ; latch data
Deassert strobe                         ; data is now latched
Do N times
       Raise clock                     ; clocks in 1 bit
       Lower clock
       Read in data bit                        ; 'nuff said
       Add data bit to stored data.    ; same.
End Do

Output is about the same except that you provide a latch pulse
after all data has been output to the serial chain.
This process is fast enough for most applications and is able
to be interupted etc safely and with ease (unlike eg bit bashed
RS232 serial i/o).

The terminally clever could do this on 2 or possibly eben only
1 pin, perhaps. I have thought about it but have not (yet) had
the need to do so. An analog time delay would allow strobe and
clock to share the same pin (long oulse is strobe / short pulse
is clock).

The 4021 and 4094 are available in surface mount which is
achievable technology for an amateur (I don't know your
application).

An alternative possibility is to use analog inputs if your PIC
variant is so equipped and provide a weighted resistor chain
from the BCD switches. 4 resistors can easily be read on a
single pin (1 BCD thubwheel) and you may manage 8 with care
using an 8 bit A2D.
Using a single A2D pinand a few diodes you could feed each
thumbwheel common in turn with a digital high and read the
analog resultant. This would probably take (out of head so may
be wrong) 3 digital and 1 analog pin plus at least a commoning
diode per BCD switch and possibly diodes for all output lines
(12).

       =====================================



*       ST6 - the world's second most RISC microprocessor (that I
know of) :-).
       The PDP8 with 8 instructions is the first (as far as I am
aware)..

1997\08\21@112506 by Lynn Richardson

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At 03:43 PM 8/20/97 +1000, you wrote:
>Maybe I've missied something, but, is it not easier (no diodes and
>standard shift-register) to use the shift-register to shift IN the
>switch positions.
>
>In a normal setup the BCD outputs of the switch present a logic 1 or 0.
>Therefore, if they are connected to the 12 parallel shift register
>inputs, the value of each input would be serially shifted in.
>


Much better! Remember to put in 12 pull down resistors from the switch
outputs to ground (if the switch commons are tied to +5v).  Then use 8 bit
parallel in/serial out 74HC165's. They cost about $0.33 each in 100s.


---------------------------------------------------------------------------
Lynn Richardson - Design Eng.|WA0ZNL            |Progress Instruments, Inc.
DC - 1GHz, RX, TX 100W, PLL  |WA0ZNL.AMPR.ORG   |807 NW Commerce Drive
ASM 6805, 8051, Z8, PIC      |44.46.176.3       |Lee's Summit, MO 64086
C                            |lrichSTOPspamspamspam_OUTproginst.com|P(816)524-4442 F 246-4556

1997\08\21@113104 by Mike

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At 11:43 PM 8/20/97 -0800, you wrote:
>Mike <spamBeGonePICLISTSTOPspamspamEraseMEMITVMA.MIT.EDU> wrote:
>
>> WHy not treat the thumbwheels as if they are keys annd use diodes,
>> selecting the group via one of three port pins -  Ah la HC139 etc ?
>
>Mike:
>
>That's the "seven pins and twelve diodes" method that was mentioned
>in the original question.  The diodes are necessary because ALL
>combinations (from no sitches closed all the way up to all 12
>switches closed) need to be unambiguously read.

Yes Andy but, thats 6 pins. I think I prefer the dual HC165 method
instead since its only 3 pins - yet 16 bits of input, no diodes and
only a couple of cheap 100K SIPs...

Rgds

Mike
perth, Western Australia

1997\08\21@121839 by Harold Hallikainen

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On Thu, 21 Aug 1997 22:56:14 +1200 Russell McMahon <KILLspamapptechspamBeGonespamCLEAR.NET.NZ>
writes:

>*       ST6 - the world's second most RISC microprocessor (that I
>know of) :-).
>        The PDP8 with 8 instructions is the first (as far as I am
>aware)..
>


       My first teaching job was teaching PDP-8 assembly language.  We
started with programming it in binary using the front panel switches,
then moved on to the two pass assembler, which meant we had to run our
source code paper tape through the reader twice.  We then got our object
code out on more punched paper tape.  It had a clever way of handling
subroutine return addresses without a stack.  The return address was
stored in the first location of the subroutine.  Code started in the
second location of the subroutine.  A return instruction did an indirect
jump through the first address.  Worked great except that you couldn't do
recursion or ROM your code.  Of course, this was running on core, so I
guess ROM was not an issue...

Harold

1997\08\21@214955 by Tom Handley

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  Alec, I've seen several good solutions mentioned but another you might
consider is the CD4067 16 channel analog multiplexer. It would require five
lines though. Four for address and one for data. It also has an enable input
so you could cascade them for more switches.

  - Tom

At 10:43 AM 8/20/97 PST, you wrote:
>     From:    Alec Myers <EraseMEAlecspamEraseMESHRUBBERY.DEMON.CO.UK>
>
>     Does any know a cute way of reading three BCD thumbwheels that uses
>     less than
>             i)      Twelve i/o pins
>             ii)     Seven pins and 12 diodes?
>

1997\08\21@224051 by Michael S. Hagberg

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how many i/o lines do you have to spare?
how much board space is available for components?
how many boards are going to be produced?

these are some of the questions i ask myself when designing projects. then
i consider the following solutions.

this could be done with one i/o line and about $3.75 USD in components
OR three i/o lines and about $1.00 USD in components
OR seven i/o lines and about .10 USD in components
OR twelve i/o lines and no additional components

Which one best fits your current project?

michael

>At 10:43 AM 8/20/97 PST, you wrote:
>>     From:    Alec Myers <@spam@Alec@spam@spamspam_OUTSHRUBBERY.DEMON.CO.UK>
>>
>>     Does any know a cute way of reading three BCD thumbwheels that uses
>>     less than
>>             i)      Twelve i/o pins
>>             ii)     Seven pins and 12 diodes?
>>
>
>

1997\08\22@091548 by Alec Myers

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>
>this could be done with one i/o line and about $3.75 USD in components
> OR three i/o lines and about $1.00 USD in components
> OR seven i/o lines and about .10 USD in components
> OR twelve i/o lines and no additional components


Thanks for all the suggestions:

I've seen the three pin A-D solution, with resistor networks, but it used
1% tolerance resistors in various odd values - as someone's pointed out
recently this is unhelpful froma manufacturing point of view.

I hadn't thought of the shift register idea though. I suppose that actually
a '554 or '61 (with 13 i/o pins) could read twelve pins and clock the data
out of the thirteenth in some kind of slow async. serial protocol - and it
can sit and do this 100% of the time. So only 1 pin is needed on the
primary PIC. And it might as well do the BCD-binary conversion while it
sits there.

And while I sit here writing this, it's just occured to me that there's a
remote-control IC (HS12E I think) that takes 12 binary inputs and clocks
them out in an odd but decipherable PWM format. That might be the cheapest
single pin solution.

1997\08\22@092606 by Pasi T Mustalahti

picon face
On Fri, 22 Aug 1997, Alec Myers wrote:

> >
> >this could be done with one i/o line and about $3.75 USD in components
> > OR three i/o lines and about $1.00 USD in components
> > OR seven i/o lines and about .10 USD in components
> > OR twelve i/o lines and no additional components
>
>
> And while I sit here writing this, it's just occured to me that there's a
> remote-control IC (HS12E I think) that takes 12 binary inputs and clocks
> them out in an odd but decipherable PWM format. That might be the cheapest
> single pin solution.

PTM: How about MC14457, MC14458, MC14497 or equivalent remote controller,
if you find some. connection with just one pin, 'look ma no hands'.

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1997\08\26@041100 by Keith Dowsett

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face
At 22:20 19/08/97 -0800, you wrote:
{Quote hidden}

There is another possibility. Why not read all the pins into a shift
register (IIRC one or more 74LS299 will do this) then shift it into the PIC
from there. No Diodes, but required 3 pins (read-data, clock and data.)

Keith.
------------------------------------------------------------
Keith Dowsett         "Variables won't; constants aren't."

E-mail: TakeThisOuTkdowsett.....spamTakeThisOuTrpms.ac.uk  or TakeThisOuTkdowsettKILLspamspamspamgeocities.com

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'BCD Thumbwheels'
1997\09\05@083931 by Larry G. Nelson Sr.
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face
If you use 1 BCD per A/D channel the tolerences are not a problem. If you
wanted to do two per channel then that would be pushing it and not
manufacturable. One client is using this technique without problem in a
consumer product at very high volumes.


At 12:40 PM 8/22/97 +0100, you wrote:
{Quote hidden}

Larry G. Nelson Sr.
.....L.NelsonspamRemoveMEieee.org
http://www.ultranet.com/~nr

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