of Microprocessors

ORGAN KEYBOARD INTERFACE 307
with the upper bus is remade the velocity count is reset. Finally, however,
the bouncing stops and the State 1 looparound increments the velocity
counter on each scan cycle. The very first time contact with the lower bus is
seen, a press event is generated using the accumulated velocity count. Thus,
the desired timing from last upper bounce to first lower bounce is implemented.
An exactly analogous sequence occurs when the key is released.
Note that simultaneous contact with both upper and lower buses is physically
impossible and therefore not accounted for in the diagram.
Program Description
A complete, assembled program listing for the keyboard is given in
Fig. 9-9. Looking at RAM memory allocation first, it is seen that nearly half
of it is devoted to saving information about the state of each key. The low 2
bits ofeach byte are used to hold the state number, while the upper 6 bits are
used as a velocity counter. The entire state byte is 0 if a key is inactive.
Thirty-two more bytes are used to implement an event queue, which allows
up to seven events to be stacked up before overflow. Five bytes are used to
0001 0000
0002 0000
0003 0000
0004 0000
0005 0000
0006 0000
0007 0000
0008 0000
0009 0080
0010 0081
0011 0082
0012 0083
0013 009C
0014 008C
0015 0100
0016 0000
0017 0400
0018 0440
0019 FCOO
0020 0000
0021 0000
0022 0000
0023 0000
0024 0000
0025 0030
0026 003E
0027 003F
0028 005F
0029 0062
0030 0064
0031 0065
0032 0067
0033 0067
0034 0067
0035 0067
PORAM
PAREG
PAOIR
P8REG
P8DIR
TMWRI
TMRD
P1RAM
MKYBU
MKYBL
ROM
MKYBST
OUIP
QUOP
EVQU
TIME
TIMEF
TIMCNT
VCJPPT
MUSIC KEYBOARD CONTROL PROGRAM
THIS EXAMPLE PROGRAM SCANS A 61 NOTE VELOCITY SENSITIVE MUSIC
KEYBOARD AND FORMATS THE SIGNIFICANT EVENTS INTO 4 BYTE GROUPS
FOR USE BY A HOST MICROCOMPUTER SYSTEM.
DEVICE AND MEMORY ADDRESSES
$0000
$0080
$0081
$0082
$0083
$009C
$OoaC
$0100
$0400
$0440
$FCOO
RAM MEMORY ALLOCATION
*= PORAM
*=*+ 61
*=*+ 1
*=*+ 1
*=*+ 32
*=*+ 3
*=*+ 2
*=*+ 1
*=*+ 2
IN ITI ALI ZATI ON
*= ROM
FIRST PAGE 0 RAM LOCATION
6532 I/O PORT A DATA REGISTER
6532 I/O PORT A DIRECTION REGISTER
6532 I/O PORT B DATA REGISTER
6532 I/O PORT B DIRECTION REGISTER
6532 TIMER WRITE, /1, ENABLE TIMER INTERRUPT
6532 TIMER READ, LEAVE INTERRUPT ENABLED
FIRST PAGE 1 RAM LOCATION (IS SAME 128
BYTES AS PORAM
BASE ADDRESS OF MUSIC KEYBOARD UPPER BUS
BASE ADDRESS OF MUSIC KEYBOARD LOWER BUS
FIRST LOCATION IN lK EPROM
SPACE FOR THE CURRENT STATE OF 61 KEYS
EVENT QUEUE INPUT POINTER
EVENT QUEUE OUTPUT POINTER
SPACE TO QUEUE UP TO 7 EVENTS
ELAPSED TIME IN MILLISECONDS SINCE TURNON
FRACTIONAL PART OF ELAPSED TIME
5 COUNTER FOR TIMER INTERRUPTS
INDIRECT POINTER FOR VECTOR JUMP
; START AT BEGINNING OF ROM
Fig. 9-9. Listing of keyboard control program