At 04:46 PM 2/22/00 -0500, Bill wrote:
>I have an embedded computer which has four user
>readable DIP switches on it. Board is running
>at 3.3 volts. I want to be able to distinguish
>between 7 possible SPST switches being closed
>(one at a time) or all being open. I thought
>"OK. I'll just convert the single switch event
>to a binary number by hooking it to the appropriate
>pins on the DIP switch. Then I'll prevent the
>multi-bit numbers from cross feeding the single
>bit events by putting diodes in." But, the diodes
>have too much resistance for the board to see the
>switch as closed. The single-bit switches, 1, 2
>and 4, read just fine.
The diodes aren't your problem. The circuit is. Even 1N914 "glass" diodes
that you buy at radio shack will conduct at 3.3v. You are doing one row of
a keyboard matrix encoder consider the following lousy ASCII diagram.
The "*" are connection between wires, the D is a Diode pointed "down" and
to the right. All inputs are presumed to be pulled up to Vcc when open
circuited. Note that since you are using "ground" as your reference you
can't detect switch #7 (only switches zero through six, although they can
be silk screened 1 through 7 :-) The logic must detect .7v as being logic 0
(usually not a problem)
Bit 0 ------*-------------*-------------*-------------*----
\ | | \ | | \ | | \ |
D | | D | | D | | D |
\| | \| | \| | \|
* | * | * | *
| | | | | | |
Bit 1 ----------|--*---|--*---|------|------|--*---|--*---|
| \ | \ | | | \ | \ |
| D | D | | | D | D |
| \| \| | | \| \|
| * * | | * *
| | | | | | |
Bit 2 ----------|------|------|--*---|--*---|--*---|--*---|
| | | \ | \ | \ | \ |
| | | D | D | D | D |
| | | \| \| \| \|
| | | * * * *
| | | | | | |
| | | | | | |
/ / / / / / /
/ / / / / / /
| | | | | | |
Ground ---------+------+------+------+------+------+------+
Switch # 7 6 5 4 3 2 1
--Chuck
Received on Tue Feb 22 2000 - 17:31:14 GMT
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