Keyboard Controller; Signals Around The 8253 - Sharp MZ-700 Service Manual

c) Keyboard controDer
The 8255 writes strobe (key scan signals) on PA and
reads key data from PB. The table shown below is the
key map.
d) Signals around the 8253
The 8253 Programmable Timer generates the buzzer tone
through the counter
#(J
and keeps the internal timer
function via the counters #1 and #2.
The counter
#(J
is used as a square waveform generator
(Mode 3). The counter #1 is used as a rate generator
(Mode 2) and #2 as an interrupt upon terminal count
(Mode 0).
The counter
#(J
counts the input pulse of 895KHz which
is divided by a predetermined factor (musical note), and
is then supplied to the amplifier to generate sound. The
counter #1 receives an input pulse of 15.6KHz and creates
a pulse on OUTl every second. The counter #2 counts
those pulses and OUT2 turns to a
high
level 12 hours
after. As OUT2 is connected to the CPU interrupt pin,
it then goes into an interrupt processing routine.
8253
D7
Do
14
OUT2+----TO CPU INT
OUT1
ClK1
BlNK 05.6KHz)
OUTO
sp,
ClKO
895KHz