4
The touch control was imple-
mented with toggle switches
mounted on the keys. The time
needed for toggling was
detected as a measure for
the power of the keystroke.
The particular challenge
was, that the switches had
to be scanned with at least
1 kHz for an appropriate
resolution, but the 1 MHz 6502
CPU was too slow to perform
the scan of 76 keys.
We solved that problem with
a self developed controller
which scanned the keyboard and
put op-codes on the CPU's data bus,
in realtime: a "NOP", when a
key didn't need any attention,
a call to the suitable subroutine
(e.g. increment of counters)
otherwise.
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