18 Sep
2007
18 Sep
'07
7:46 p.m.
Somewhere I think I have a bare keyboard (no keycaps
or case, but real=20
switches as opposed to the junk plastic sheets you get with some these da=
ys), =20
and it has a ROM-less part in there and an eprom next to it.
I've seen many keyboards where the PCB was laid out to take the EPROM
(and an address latch in most cases). There was a jumper that selected
whether the microcontroller would use its internal ROM or the external
one. So you could use a mask-programmed microcontroller, an EPROMed one,
or a ROMless microcontroller with a separate EPROM alongside it.
Somewhere I have a manual for a keyboard, back when things came with real
manuals, which, while it doesn;'t contain the source listing for the
firmware, it doess say 'the normal keycode table starts at address
<xxxx>, the shifted keycode table at address <yyyy>' etc. And there's
a
table of the keys and the offset into each table. It appears you were
expected to modify the keycodes... Of course such a manual also cotnains
a schematic.
-tony