of Microprocessors

SOME REAL ApPLICATIONS 727
for that transition are written to latches that the main 6809 can read. The
6809 is also interrupted so that response to the keystroke is rapid.
Each keyboard key is equipped with a double-throw contact actuated
by the rear edge of the weighted, wooden keys. The 61 keys, therefore,
constitute 122 contacts, which are wired in a 16 X 8 matrix rather than to a
bunch of digital multiplexor chips as in the Chapter 9 example. One
potential problem with any kind ofswitch matrix is sneak paths that can occur
when three or more switches are closed simultaneously. This is ususally dealt
with by placing a diode in series with each switch but is handled differently
in the Chroma.
Figure 19-8 shows a portion of the keyswitch matrix. The 8039
outputs a 4-bit "switch bank" number and reads back the state of eight
switches in the bank. When a switch is closed, it draws a 0.5-mA current
through its series 10K resistor and a 100-ohm sense resistor. The positive
inputs of the comparators are all biased at 25 mV below + 5V, whereas a
closed key develops a 50-mV drop across the 100-ohm sense resistor. Besides
limiting the current, the 10K series resistors prevent the formation of any
significant sneak paths. The LM339 quad comparators are exceptionally
cheap but are relatively slow at 1. 2 /-Lsec, requiring a wait state to be added
when the 8039 reads a switch bank. Although this matrix arrangement is not
as straightforward as using digital multiplexots, it does keep high-amplitude,
high-speed digital signals out of the physically large keyboard contact
atea where they might generate unacceptable radio frequency interference.
Control Program and Panel Function
The most critical design aspect of a mass-market keyboard synthesizer
IS the control interface it presents to the user. In the Chroma, this is
determined by the physical panel controls available and the logical structure
of the control program. Unfortunately, a complete descriprion of the user
interface would require a chapter by itself so it will be very briefly
summarized here.
Most panel operations revolve around the 50 numbered buttons, the
two-digit and eight-digit displays, and a single "parameter value" slide pot.
A few other buttons and lights are used to change operation modes and
perform specific functions, such as setting a keyboard split or tuning. One
impottant characteristic is that keyboard notes may be played at any time
regardless of the panel mode. The only exception is during tuning or cassette
dump/load operations.
There are two primary operating modes of the control panel. In the
"program select mode," each of the 50 numbered keys is associated with a
different "patch" or "sound." Simply pressing a button will instantly select
the new patch for all subsequent notes, while silencing any that may already
be sounding. Thus, during performance, different sounds may be selected