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21
1.
3.5.2.
Interrupt controller
The
interrupt controller
activates
the microprocessor
interrupt
input
facility
to allow
certain
subroutines
to
be performed
either
by manual
or
automatic
control.
The
control inputs are interrupt pulses
applied
from
either
one
of
the four
front-panel
mode
switches or
from
timing
pulses
generated
from
the
comparator
output
in
the timing
mode.
a)
Mode
switch
interrupts
The
logic
0
required for interrupt
is
derived
from one
of the four
mode
pushbuttons.
It
provides
for:
UP
and
DOWN
ranging
in
the
manual
mode,
AUTO
ranging (except
in
timer
function).
—
Trigger
mode
polarity selection
—
Selecting
relative
reference
with
ZERO
SET
in Vtt.,
dB, 12,
and
°C
range
—
Manual
STOP/RESET
in
timing
mode
The
functional
circuit
for interrupts given
by
the
mode
switches
is
shown
below:
ZERO SET
Normally
the three inputs to the
NAND-gate
are at logic
1,
and
the
double
inversion
of the
NAND-gate
-I-
the
inverter gives
a
logic
1
at
the
INT
input of the microprocessor;
i.e.
no
interrupt
is
given.
However,
when
one
of the four pushbuttons
is
depressed
momentarily,
a logic
0
pulse
is
given to
the
middle
input
shown, which
results
in
a logic
0
being applied to the
INT
input,
the condition for
inter-
rupt.
b)
Trigger
level
interrupts
In
the timing
measurement mode,
interrupts are
generated
by
the
trigger
level
circuit via
the
comparator
for starting
and
stopping the
pP
internal
counter during time measuring.
In
the
normal
trigger
mode, by
the
use
of delays
and
exclusive-OR
gating, logic
0
outputs
are
generated
on
the
positive-going crossings of
the
trigger
level;
i.e.
an
interrupt
is
given
at
the
leading
edges of
successive
comparator output
pulses
to
start
and
stop the
juP
counter.
In
the
special trigger
mode,
a logic
0
applied
from
the data hold probe switch enables timing of the duty-
cycle
of an input
by
stopping the timing period
at
the negative
crossings of
the
trigger
level.
In this case,
the leading
edge
of
the
comparator output
pulse
starts
the
juP
counter
and
the
trailing
edge stops
it.
When
the
negative
trigger
level
is
selected,
the
start/stop
polarities will
be
the
reverse.
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