Hi Joe!

> Here's a simple practical keypad example -
>
> https://www.flickr.com/photos/97814409@N04/29128519532/in/album-721576729=
72293475/
>
> I've seen other resistor ratios. My voltages are perhaps a little closely
> spaced but it's reliable. The minimum spacing is 0x11 between the '7'
> and '4' buttons, which is easily discernable by the ADC, even when
> using Vdd and Vss as Vref

Thank you for the share. I'll have to spend a bit of time unwinding
your values. Since I'm not using a matrix there is the potential for
more than two resistors to combine so I'll need to make sure I don't
have any overlap in values.

> A 10-bit switch is getting quite fine though, particularly if the ADC is
> 10-bit. As Jean-Paul suggests, this would suit R-2R thinking, although
> the precision of the resistors becomes more important as bit-width
> increases
>
> https://en.wikipedia.org/wiki/Resistor_ladder
>
> "resistors of 1% accuracy would suffice for a 5-bit circuit .... For a 10=
-
> bit converter, even using 0.1% precision resistors would not guarantee
> monotonicity of output"
>
> So 2 x 5-bit is going to be the least troublesome

Ok, good to see my thoughts aren't totally wacko. My current plan is
to look at Bob's values and adjust as needed. I am trying to avoid
going true R-2R as space is incredibly limited (I'm using a micro DIP
switch that's only 9mm wide for 8 bits!), so I won't have room for the
"R" values. Bob used specific values for each pin, and I'll do
something similar I'd imagine. I think separating into two 5 bit reads
makes the most sense. I am not so short that I can't spare that pin,
it's just the 8 more needed to get a true 10 bit digital input won't
work. And no more room for a shift register or anything fancy...

At the moment I'm considering moving to 0402 resistors (gulp)! Amazing
how my "fun" projects tend to push me harder than my more "real"
projects!

Thank you both for the thoughts!

Josh
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