Tandy 1000
Timer Function
Technical Reference Manual
The Timer
is
an
8253 Timer/Counter consisting of three independent counters.
The clock for
all
three counters
is
1.1925 MHz. The gate for counter #0,
#1
is
permanently
"on".
The gate for counter #2
is
controlled by a bit of the 8255
Port 8. The output of counter #0
is
dedicated to system interrupt #0 (8259
IRO)
for software timing functions. The output of counter
#1
is
dedicated to
the REFRESH function. When the optional DMA/Memory board
is
installed,
DMA channel #0
is
used for refreshing the RAM memory. Counter
#1
sets
RFSHRO*
(DROO)
every 15 microseconds
to
initiate a single "dummy" memory
read. The output of counter #2
is
routed to the sound circuit and into the 8255
Port C for monitoring by the CPU. See Figure
9.
Sound Function
The sound function consists of
an
internal and
an
external sound circuit. The
internal sound circuit
is
directly connected
to
the speaker via U118.
The
source
of the sound frequencies
is
U96 Complex Sound Generator. Internally, U96
has four programmable sound generators. The frequency and output level of
each
is
controlled by software. The four internal generators are summed with
an
external input into a single output. The external source
is
from the 8253
counter #2 (programmable frequency and fixed amplitude).
In
addition to being
the only source for the unit speaker,
it
is
one of three selectable sources for
the external audio out signal. This signal
is
intended
as
an
input into a external
amplifier such
as
a stereo. The three sound frequency sources are:
1.
Complex sound generator U96.
2.
The 8253 counter
at
channel
2.
3.
Any external source applied to bus interface pin 808, Audio
In.
These are selected by
an
analog multiplexer U105. Selection signals are
SNDCNTLO, SNDCNTL1 from the 8255. The output driver for Audio Out
is
U119 which
is
designed to drive
3.
load impedance of 1000 ohms.
See
Figure
10.
31
To Next Page
Loading...
Need help?
General
