of Microprocessors

300 MUSICAl ApPLICATIONS OF MICROPROCESSORS
For convenience in this design example, each keyboard event will consist
of four 8-bit bytes. The first byte will be the key identification, which
will be a binary number between 0 and 60 with 0 corresponding to C l-and
60 corresponding to C6 on the five-octave 61-note keyboard. The most
significant bit of this byte will distinguish press events from release events
with a 1 signifying depression. The second byte will be a velocity value in
which small numbers correspond to high velocities (short "float" times). The
third and fourth bytes will be the time of day in I-msec increments. Although
this only pins down the time to within 65 sec, that should be quite
close enough to avoid confusion. If desired, a fifth byte could extend unique
timing information to over 4 hr.
The keyboard interface will present these four bytes as 32 parallel bits
that can be read by the using system in any way it sees fit. However, it is
extremely important that a handshake protocol be defined so that there is no
risk of an event becoming lost because another event occurred before the
using system could read the first one. This can be accomplished with a
request/response flip-flop similar to the one used with the analog keyboard.
When the keyboard has an event ready, it updates the 32-bit parallel output
and sets the request flip-flop. The using system seeing the request on, reads
the 32 bits and resets the request flip-flop. The keyboard is prevented from
outputting new data while the flip-flop is on. If another event does occur
6502 07 f-,--t"'-------,-----------,-----e---- ~~~A
+5V
10 k
NMI
ROY
SO
16 BITS
16110
LINES
TO FIG.
9-7
10
c:)----------'-----+----'-+---'----_:B~RESS
A80
R/W/-------------f-------'
1 MHz PH0 IRQ10-----------........-"""-0() +5 V
RES PH2
SYSTEM
CLOCK
3K
Fig. 9--4. Keyboard interface microprocessor