November
1997, Issue 88
From
the Bench:
Nonintrusive Interfacing - Using Kid Gloves
TEXT
AND GRAPHICS FRAMES
A
6 x 6 block of pixels is of sufficient size to hold
any alphanumeric character (including a blank column
for interletter spacing or a blank row for interline
spacing). Many letters require only four columns.
The
40 x 30 pixel screen can therefore hold about 40 characters.
Since the frames are drawn as a unit, character size
can be mixed and can freeflow on the frame.
As
a point-of-sale display, the animation can be used to
automatically scan through any of the 12 frames of graphics
in any 96-frame sequence. Or, the Animator may be commanded
to increment the frame using a variety of interframe
pauses based on the material displayed. This option
gives text-based screens more display time.
To
prevent the Animator from powering down into a power-saving
mode, a command must be sent at least once every couple
minutes. Various keys can be used (none of which will
alter the presently displayed frame).
FRAME
INPUT MODE
Although
the Etch-A-Sketch Animator is capable of storing 12
different frames, like its ancestor, your work is gone
forever once you erase a frame to draw a new creation.
Frame input mode using the Domino's serial input port
can accept 30 strings of 40 characters each.
Using
a text editor, you can create a file that uses "."
(period) and "X" to signify an erased and
drawn pixel. A carriage return at the end of every row
delimits the line.
You
can quickly see and edit the image, and then--voilą--it's
saved permanently as a file. The file can be loaded
into the Domino and placed into any of the Animator's
12 frames. A serial EEPROM may be used as local nonvolatile
storage.
JOY
STICKING
Because
the Domino has an optional two-channel ADC, an x-y
joystick can be connected for better local control.
Apply +5 V across the joystick's potentiometers, and
route the wipers into the two A/D inputs.
The
A/D conversion values are used to decide which direction
pulses to send to the Animator. Now, this is more like
playing a video game--unless you remember Pong.
So,
what did I accomplish? For the most part, it was an
exercise in interfacing--the nonintrusive control of
an established system.
At
some point, you may be asked to provide additional functions
to a system where you're not allowed to alter the operation
of an existing system. Redesigns are not always possible.
And
as an engineer, you must be ready to accept the challenge--no
matter what the constraints. Even if they come from
a nine-year-old.
