.
TANDY COMPUTER PRODUCTS
Timer Function
The Timer
is an
8253 Timer/Counter consisting
of
three
independent counters. The clock for all three counters
is
1.19
25
MHz. The gates for counters
#0
and #1
are
permanently "on". The gate for counter
#2 is
controlled
by
a bit
of
the keyboard interface (8255 Port
B).
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 RFSHRQ* (DRQO)
every
15
micro-seconds
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 10.
Sound Function
The sound function consists
of an
internal and
an
external
sound circuit. These are directly connected
to
the
Headphone Output via U5. The source of the sound
frequencies
is
U19,
a
Complex Sound Generator. Internally,
U19 has three programmable tone generators and
a
noise
generator. 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).
It is
one
of
two selectable
sources for the external audio out signal. This signal
is
intended as
an
input into
an
external earplug. The two
sound frequency sources are:
1. Complex sound generator U19.
2.
The 8253 counter
at
channel
2.
The output driver for Audio Out is U5. See Figure 11.
32
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