From: Jonathan Kirwan
Subject: Re: Keypad keys per IO pin
References: <firstname.lastname@example.org> <email@example.com> <firstname.lastname@example.org> <email@example.com> <firstname.lastname@example.org> <email@example.com>
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NNTP-Posting-Date: Thu, 16 Jan 2003 01:22:40 GMT
Organization: AT&T Broadband
Date: Thu, 16 Jan 2003 01:22:40 GMT
On Thu, 16 Jan 2003 12:09:59 +1100, "Michael Culley"
>Can't you just connect each key to a different combination if IO pins. eg
>for 4 io lines:
>Key1 Pins 1,2
>Key2 Pins 1,3
>Key3 Pins 1,4
>Key4 Pins 2,3
>Key5 Pins 2,4
>Key6 Pins 3,4
That is the simplex connection, in fact. See:
| o o |
o o o o
| / \ |
| / \ |
| / \ |
I apologize for the weird square. But it gets the point across.
It's exactly what you are saying... connecting each key to a
different combination of pins.
That's a simplex.
>If N is the number of IO lines then this gives (N-1)! keys, so for 6 pins
>you get 120 keys
No, you get (N^2-N)/2 switches. Not (N-1)!. Unless you are
talking about something else and not the simplex. But your
example appears to illustrate that you *do* mean the same thing
I do. And if that's the case, then it is (N^2-N)/2. So the
computation is (6^2-6)/2 = (36-6)/2 = 30/2 = 15.
Perhaps you need to illustrate what you actually mean with 6
I/O, if it is different. NCSRadio mentioned a web site which
hinted at a different, binomial approach which uses diodes to
get additional entries in the "symbol space." Nicely.