8 May
2004
8 May
'04
1:49 a.m.
Oh, it's a Kriz tablet (after Stan Kriz at 3
rivers).
I have never fully understood how they work either.
The 'wires' in
the PCB carry electrical currents, and are sequenced in some way by
the microcontroller. The puck/pen contains a pickup coil, the output
of which is amplified and fed back to the microcontroller.
[...]
And the resolution is _much_ higher than the spacing
between the
'wires'.
Much. The manual documents resolution settings as high as 20 counts
per mm; experimenting with it, I find it actually includes settings
that appear to attempt 1000 lines/inch and 1000 lines/mm, based on
dividing full-scale counts by pad size (in-my-head estimates; I could
be out by a factor of two or so). Of course, I doubt it's accurate to
anything like ?1 count at those settings. :-)
Actually, it's not accurate to ?1 count even at the documented
settings. I find that when I turn on stream mode and don't move the
puck, I get a stream of identical samples. If I turn on request mode
and request reports (at human typing speeds), I get identical reports -
but they are a few counts different from the reports I get in stream
mode. I conjecture that there is some kind of residual carryover when
doing two readings in quick succession, as stream mode does....
One day I'll disassemble the microcontroller ROM
and make sense of it
all...
Mine has a 27xxx-series UVEPROM (2732, I think it was), and it's
socketed. I'm very tempted to pull it and dump it, and see how much of
a schematic I can extract from the pcb (the only real problem is
component-side etch runs that run under components).
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