My means is No use IC to do it!
    only use the diodes to do it? but I don't know how to connected it?


There are two different ways that I know of.

1) You would use a 16-to-4 line encoder IC.  The output would be binary
encoded to a single key but is the simplest if you don't need to look at
more than one key at a time.

2) if you need to look a more than one key at a time, use a 16 input
shift register (port expander).  Cycle thru all 16 inputs (one at a
time) and see which bits are set.  Disadvantage to this is the time it
takes to cycle thru all 16 inputs.  Not good for fast moving signals.

Wing Kent Kwan wrote:

> how can I only uses 5 cpu I/O pin for 16 keyborad keys input?
> THANK!!!!!!!


On Sat, Mar 25, 2000 at 10:47:41PM +0800, Wing Kent Kwan wrote:
> how can I only uses 5 cpu I/O pin for 16 keyborad keys input?
> THANK!!!!!!!

      ---------       |
OUT1--          Q0--- |
                      |
OUT2-- DECODER  ...   |MATRIX
        8-OUTS        |OF
OUT3--          Q7--- |SWITCHES
      ----------      |
                      |(8 rows, 2 columns)
IN1-------------------|
                      |
IN2-------------------|
--
                        HTH
                        Wojciech Zabolotny
                        http://www.ise.pw.edu.pl/~wzab

http://www.debian.org  Linux - free OS for free people!


Three pins to a hc138 demux each pin of the 138 to two buttons on two data
lines 16 buttons and 16 diodes three outputs two inputs 5 i/o pins

Easy...........

Cheers Steve........
Ps you can do it with less if use shiftt registers......



David VanHorn wrote:

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> >> Does anyone of you knows how to measure the phone line
> >> impedance?
>
> Sure, build an impedance bridge.  Two known resistances, precisely matched,
> a variable, and the unknown. Feed a tone in across the bridge, measure the
> imbalance, tune the variable to minimize the imbalance, measure the
> variable, that's your impedance.
>
> Doing this on a live line presents some special considerations. You might
> be better to get the spec, and design to that.
>
> >> I have a voice multiplexer project and I interface to
> >> Private Automatic Branch Exchange. I can't balance the
> >> transmit and receive gain of my system. I suspect it
> >> depends on the phone line length and impedance.
>
> It probably does. Or more precisely, your return loss depends on how well
> you match your impedance to the line.
>
> I'm guessing the PI phone system uses US standards, which would be 600 ohms
> nominal, with some reactance, but I forget. It's been a few years since I
> dealt with this.
>
> If you have a flexible hybrid design, a microcontroller, and a way to
> measure signal levels, then you can have the system calibrate itself by
> injecting a tone on the transmit side of the hybrid, and measuring the
> level on the receive side of the hybrid, then tune a digital pot for
> maximum return loss. You should only have to do this once. It's certainly
> not something you want to do on every call!
>
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