For
this
example,
let's
follow
what
happens
to
the
display
at
power-up.
Refer
to
the
block
diagram
in Fig.
5-3 while you
read
the
following
text.
The
display
logic
works
on
the
basis
of
having
binary
values
(representing
every
character and
every display position)
stored
in
display
RAM.
This
value
is
fed
through
a
character
PROM
which converts
the
binary value
into
signals
that
can
be used
by
a
7-segment
LED
display.
Whenever the
value
in
display
RAM
is
changed, the 7-segment
readout
is
changed.
The
display
logic
is
self-refreshing
— that
is,
it
doesn't require
any
external
signal to display
a
character.
Whatever
is
in
the
display
RAM is shown
on
the
LEDs.
In
order
to
place a new
character
on
the
display, a
newbinary
value
must
be
written
into
the
display
RAM.
At
power-up,
there
is
no way
of
knowing
the
contents
of
the
display
RAM.
Therefore, the first thing
the
MicroLab
|
monitor
must
do
is
fill
the
display with blanks,
by
writing
a
binary value
that
stands
for
“blank” to each display
RAM
location.
Write
Enable
ONBIO
Display RAM
Theory
of
Operation—MicroLab
|
Instruction
To
the
microprocessor, the display
logic
exists as
an
1/0.
address.
So,
the
microprocessor
places
the
|/O
address
of
the
display on
the address
bus.
The
microprocessor
then
places
the
value
for
“blank” on
the
data
bus.
This
action
is
repeated
for
each
display memory location. The result
—
the
display
is
blanked.
To
place
the greeting message
("HELLO")
on
the
display,
the
microprocessor
goes
through the
same
routine
as
it
did
to blank
the
display. The microprocessor
places
the
display
\/O
address
on
the address
bus, and
then writes
the
characters
into
display
RAM.
Every
operation
within the
MicroLab
|
works
in
essentially
the same
way.
If
the
microprocessor
needs
to
get
information out
one
of
the
serial
ports,
the
port !/O
address
is
placed on
the
address
bus,
and
information
is
passed
to
the
port on
the
data bus.
As you
can
see
in
Fig.
5-2, the
MicroLab
|
bus
is
central
to
all
other
logic
Next,
we'll
discuss
in
detail each
block
of
logic
within
the
MicroLab
|.
To
LED
Drivers
Character
ROM
Read Enable
Display
and
Character
Counter
Fig.
5-3. Keypad/Display
Block
Diagram.
This
diagram shows the
logic
found
within
the keypad/display
block of
Fig.
5-2.
®
5-3
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