more chording keyboards

From: Eric Smith <eric_at_brouhaha.com>
Date: Thu Apr 22 16:29:26 1999

> I've always wondered how you make a keyboard that has two-key rollover, or
> N-key rollover for that matter. I know that most keyboards these days scan
> a matrix of switches, but I don't know where rollover comes into that process.

For the sake of discussion let's consider only common keyswitches, and not
capacitive or hall effect devices.

Two key rollover is easy. As long as no more than two keys in the matrix
are pressed, the scanning hardware (or software) can reliably detect when
each is pressed.

However, as soon as three keys may be pressed, you may have extra conduction
paths through the key matrix that look like phantom keys, so it is no longer
possible to distinguish valid keypresses.

N-key rollover is most trivially accomplished by putting a diode in series
with each keyswitch, so that there is no problem with phantom keys.

Eric
Received on Thu Apr 22 1999 - 16:29:26 BST