[sdiy] Matrix keyboard idea

Colin Fraser colinf at ntlworld.com
Mon Mar 19 14:28:38 CET 2001


> -----Original Message-----
> From: owner-synth-diy at node12b53.a2000.nl
> [mailto:owner-synth-diy at node12b53.a2000.nl]On Behalf Of Ullrich Peter
> Sent: 19 March 2001 08:40
> To: 'steve jones'; synth-diy at node12b53.a2000.nl
> Subject: AW: [sdiy] Matrix keyboard idea
>
>
> >On Scott Rider's page:
> >http://www.mcs.com/~syzygy/webdoc/mpx61.html
> >
> >he describes his design (from 1985) for providing MIDI in
> for any MATRIX
> >keyboard. Has anyone worked on this idea since then?
>
> I made a MIDI-interface like this one but with special matrix
> circuits from Mitel instead
> of the many analog switches. I use MT8816 analog matrix switches.
> I use a 89S8252 microcontroller and 4 of these matrix chips
> playced on a Euro-card.
> The midi interface is a little pcb that can be mounted on the
> backpanel of the keyboard.

I made a similar matrix spoofing interface some years ago based on a 6502,
but redesigned it recently using an AVR 90s2313.

I used 74HC374 latches with diodes added to the outputs to act as open
collectors.
This greatly reduces the chip count needed - I just used a latch for every 8
keys, and wire-ored the outputs together.
The column select lines from the scanner just enable the outputs on each
latch.

I put a photo of the board installed in my Polysix at
http://www.octavo.demon.co.uk/p6midi.jpg
I'll put the code and single sided PCB foil up before long.

It may be possible to do away with the latches altogether.
By raising an interrupt when a scanning pulse appears that detects which
column is being scanned then outputs the row bits as needed on a cpu port,
you could do the whole thing in half a dozen chips.
There are timing issues with this approach I would need to check out first -
I wouldn't want the CPU to ever miss a scan pulse.

Colin f





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