22 Jan
2012
22 Jan
'12
9:27 a.m.
On 22 Jan 2012 at 3:57, J.G.Harston wrote:
Also, it's electrically not possible to do 4-key
rollover with a
matrix keyboard, as as soon as three keys are pressed the four
key on the other corner of the "square" formed by the three
keys being pressed is seen as being pressed. There is nothing
software can do about this, it's an electrical function of
matrix keyboard.
Not strictly true! If one uses a *good* keyboard with an isolation
diode at each key, n-key rollover (where n is greater than 2) is the
result. For a scanned music keyboard, this is a necessity if chords
are to be supported. But many PC keyboards implement this also. So,
unless a non-n-key-rollover keyboard is laid out carefully,
"ghosting" can get to be very annoying for a fast touch typist who
pays more attention to the "make" of a key than the "break" (try on
an electric typewriter for example).
I'm not aware of any crosspoint switch ICs that have a diode at each
junction, but perhaps they exist.
--Chuck