of Microprocessors

302 MUSICAL ApPLICATIONS OF MICROPROCESSORS
with the analog keyboard, it is best to connect the digital keyboard to the
host computer's interrupt system so that fast response is retained without
tying up the music-synthesis program by constantly looking for keyboard
events.
Hardware Configuration
The hardware configuration of the interface is relatively simple as can
be seen in the simplified schematic of Fig. 9-4. First, of course, is the 6502
microprocessor, which could be easily replaced with a 6503 (28 pins and 4K
addressing range) for absolute minimum space and cost requirements.
Program storage is provided by a single type 2708 erasable programmable
read-only memory (EPROM), which holds lK bytes (2716- or 2732-type 5-V
only EPROMS can be used by simply grounding their additional address
lines). A type 6532 "combination" chip supplies 128 bytes of RAM, 16 bits
of1/0, and an interval timer that will be used to determine the "time of day"
that keys are pressed. The RAM will be used to hold information about the
state of each key, as a queue for events waiting to be accepted by the host, and
for general temporary storage.
Auxillary logic will be required to decode addresses, latch the 32-bit
output, and to actually connect to the 61 keys on the keyboard. The latter is
accomplished with a 61-input digital multiplexor as in Fig. 9-5. The
keyboard appears to the microprocessor as 128 memory locations at addresses
0400-047F16. The first 64 addresses, 0400-043F, select a key and the upper
keyboard bus, while 0440-047F select the same keys but the lower keyboard
bus. The data read from this pseudomemory have bit 7 (the sign bit) off if the
selected key is contacting the selecwd bus and on otherwise. The other bits are
forced to be zero, which is required for proper operation of the keyboard
program to be described. Addressing the keyboard like memory makes key
scanning much faster than it would otherwise be. A complete memory map is
shown below:
Hex address range
0000-007F
0080-00FF
0100-03FF
0400-043F
0440-047F
0480-07FF
0800-0BFF
OCOO-OFFF
100o-FFFF
Device addressed
128 bytes RAM in 6532
I/O ports and timer in 6532
OOOO-OOFF repeated three times
Music keyboard upper bus
Music keyboard lower bus
0400-047F repeated seven times
Unassigned, available for expansion
1K bytes EPROM 2708 type
OOO-OFFF repeated 15 times
Note that since address decoding is incomplete a variety of addresses
will refer to the same device. In particular, the EPROM can be reached by
addresses between FCOO and FFFF, which include the interrupt vectors. The