I should put a caveat here.  When testing a column, all other columns should be
inputs.  If you drive the other columns low, and the one column high, then two
buttons could cause you to short through the keypad.

-Adam

"M. Adam Davis" wrote:
>
> Wagner Lipnharski wrote:
> >
> > Again, obstructing light in a matrix beam system doesn't work for
> > multiple objects. is just like to use two switches per key in a matrix
> > system, one switch interrupts the rows line another interrupt the
> > columns, you would never know where are two or more objects.  Except if
> > all objects are in the same row or the same column.
>
> I can see how the laser system would have some difficulty with two objects
> placed just right, but matrix keypads don't have that bad of a limitation:
>
> You can detect at least two objects on a matrix system, such as a keypad.  If
> you have more than two then there are positions which would obscure the
> readings.
>
> I'll use R and C for row and column, and assume the software sweeps the columns
> in order.
> we'll take a 4x4 matrix as an example.  There are three possibilites with two
> simultaneous buttons: Two buttons in one row, two buttons in one column, buttons
> have seperate rows and columns
>
>   1 2 3 4
> 1 X X O O
> 2 O O O O
> 3 O O O O
> 4 O O O O
> Software sees the following:
> C1 returns R1
> C2 returns R1
> C3 returns nothing
> C4 returns nothing
>
>   1 2 3 4
> 1 X O O O
> 2 X O O O
> 3 O O O O
> 4 O O O O
> Software sees the following:
> C1 returns R1 & R2
> C2 returns nothing
> C3 returns nothing
> C4 returns nothing
>
>   1 2 3 4
> 1 X O O O
> 2 O X O O
> 3 O O O O
> 4 O O O O
> Software sees the following:
> C1 returns R1
> C2 returns R2
> C3 returns nothing
> C4 returns nothing
>
> These three cases cover every possible combination of two keys on one flat
> matrix.  In all cases the software can easily determine which keys were pressed.
>
> Three button presses causes problems if the matrix doesn't have diodes:
>   1 2 3 4
> 1 X O O O
> 2 X X O O
> 3 O O O O
> 4 O O O O
> Software sees the following:
> C1 returns R1 & R2
> C2 returns R1 & R2 (this is because the current flows through C1:R2 and is seen
> on R1 through C1)
> C3 returns nothing
> C4 returns nothing
>
> This can be ameliorated with careful key planning (ie, each key in a seperate
> row or each in a seperate column) or through the use of diodes on each key.
>
> With a laser system, though, the problem is that you can't select a row at a
> time.
>
> -Adam
>
> "I have nothing against work, especially when it is performed quietly and
> unobtrusively by someone else" - Barbara Ehrenreich