It works like a D/A with each switch being a binary count input. 1,2,4,8,...
The number represented by the switch combo is translated to an analog value
by the R/2R ladder and the A/D reads the value. You use a look up table
type of code to determine the switch combo ie if between 1.05 and 1.35 it
is switches 3 and 5 or some such value you have calculated. The problem is
in the number of switches. Theoretically you can use 8 switches with an 8
bit converter but with tolerances and noise I usually have 4 or 5 that I
set as a limit.
Larry


At 02:24 PM 4/16/03 -0500, you wrote:
>Just saw this approach, and wondered how does one know which combinations
>are presed?
>
>
>And how can one press all 4 buttons at once and get correct output?
>
>-----Original Message-----
>From: Larry G. Nelson Sr. [mailto:l.nelson@IEEE.ORG]
>Sent: Wednesday, April 16, 2003 10:02 AM
>To: PICLIST@MITVMA.MIT.EDU
>Subject: Re: [OT:] different approach to keyboard scanning?
>
>
>Another method is to use an R / 2R ladder configuration. That allows you to
>have multiple key presses or chording. There is a real limit due to
>resolution of the converter and accuracy of the components but this method
>lets you use resistor networks and the absolute value is less important
>than the match between components. 4 buttons allows 16 combinations and
>works well with a PIC 8 bit converter. I did this 7 or 8 years ago and many
>of you probably have used the product it is in.
>Larry
>
>
>
>At 10:34 AM 4/16/03 -0400, Wagner Lipnharski wrote:
> >Edson Brusque wrote:
> > > Hello,
> > >
> > >      I've been thinking about using the PIC A/D to scan a 16 keys
> > > keyboard. The example bellow have only 4 keys for simplicity:
> > >
> > >              ,----K1----VDD
> > >              |
> > >              1k
> > >              |
> > >              +----K2----VDD
> > >              |
> > >              1k
> > >              |
> > >              +----K3----VDD
> > >              |
> > >              1k
> > >              |
> > >       ADC----+----K4----VDD
> > >              |
> > >              1k
> > >              |
> > >             GND
> > >
> > >      Without any key pressed, the ADC would read 0V, with K1 pressed
> > > it would be 1.25V, with K2=1.66V, K3=2.5V, K4=5V.
> > >
> > >      I think this would be very interesting for projects using a PIC
> > > with ADC and short of I/O.
> > >
> > >      Best regards,
> > >
> > >      Brusque
> >
> >
> >Other than that, look this design we made and used 11 years ago;
> >
> >http://www.ustr.net/electronics/akeys.shtml
> >
> >Wagner.
> >
> >--
> >http://www.piclist.com hint: To leave the PICList
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>
>Larry G. Nelson Sr.
>mailto:L.Nelson@ieee.org
>http://www.mchipguru.com
>
>--
>http://www.piclist.com hint: To leave the PICList
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>
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Larry G. Nelson Sr.
mailto:L.Nelson@ieee.org
http://www.mchipguru.com

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