ivp wrote:

> Hi all
>
> I have a project which needs two timing intervals set. I did the
> proof-of-concept with a 16F88 but would like to use a 12F675
> in the actual product
>
> Using the 8 pins of the F88's PortB, each interval was set digitally
> with its own 4-way DIP switch. ie B<3:0> and <7:4> ranged 0-F
> individually. The value for each nybble is the counter for a 10ms
> timer, so Interval1 can be set from 0 to 150ms, and likewise Interval2
> also from 0 to 150ms
>
> I have two A/D pins free on the 12F and can use those to measure
> distinct voltages. That could be done with two 12-way rotary
> switches + resistors, possibly using a 12ms timer to get the same
> approximate range
>
> My preference would be to use the 4-way DIP switches initially
> and probably replace those with solder bridging pads later once
> the client has a better idea of what the optimum settings are
>
> Referring to the attached drawing, is there an algorithm to determine
> the values of the resistors so that the A/D can best discriminate a
> voltage which corresponds to the binary setting of the DIP ? Or has
> anyone done or seen this done before ?
>
> TIA
>
> Joe
>
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> *
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>
Joe,

Can you flip the circuit around so the single resistor goes to ground=20
and Vdd is sourced to the four switchable resistors?  If so, it makes=20
calculating the divider ratios easier.  Assign each switchable resistor=20
a binary weight and put this into a spreadsheet.  Then start plugging=20
standard 1% resistor values into the spreadsheet to get the output=20
values.  This will give you 16 distinct output voltages with all open=20
equal to zero.  Other than fiddling with the values, this is the easiest=20
hardware solution.  If you use the rotary switch, all you have to do is=20
equally weight all resistors since you will only have one switch closed=20
at a time.

Bill
--=20
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