of Microprocessors

ORGAN KEYBOARD INTERFACE 303
1--+---REOUEST
ACKNOWLEDGE
6532
PB7 SPARE
B
KEY
PB6 SPARE NUMBER
PBS
PB4
PB3
PB2
PBI
PB0
PA7
PA6
PAS
PM
PA3
8
PA2
PAl
PA0
KEY # DEN
Fig. 9-6. Using system interface
128 bytes of RAM in the 6532 can be reached by both page zero and page
one addresses, which allows both stack usage and convenient page zero
addressing.
The remaining logic is shown in Fig. 9-6. The four-latch les are wired
to 12 of the port bits on the 6532 as a simple output expansion (12 bits to 32
bits). The keyboard program can manipulate the data and clock inputs to the
latches and thus exert complete control over their contents. The four groups
of three-state outputs from the latches allow flexible and convenient interfacing
to the using system. Two more of the port bits are used to control and
sense the handshake flip-flop. The remaining 2 bits are available for expan­
SIOn.