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Issue
99, October 1998
X-Y Graphing Data
Logger
by
Alberto Ricci Bitti
Start
• Machine
Muscle
• Using
Componentware
• Low-Power
Guys
• Flash
Risc Glue
• The Graphic
Engine
• Casio
Protocol
• Main
Code
• Inside
The Box
• Setting
up the Casio
• First
Graph
• Modeling
The Data
• Broader
Vision
• Software,
References & Sources
MACHINE
MUSCLE
The data logger consists
of the graphic calculator—with its 64 ´ 128 LCD screen
and keyboard—and the analog interface. I left the
calculator untouched, and the analog interface fits in
a small, separate plastic box (see Photo 1). They are
joined only by the umbilical serial data connection.
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Photo 1—The
circuit fits inside a small plastic box just
as large as the calculator. The micro jack connector
hangs out, connecting to the calculator serial
port. The box is stuck to the bottom of the
calculator for operating. This arrangement gives
the calculator a stable and comfortable slope.
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The interface enables me
to read the voltage at up to eight inputs with 1-mV resolution
in the 0–4096-mV range, thanks to a 12-bit ADC. The
data is converted to the right protocol by a PIC microcontroller
and sent to the calculator for display and storage.
Once the data is received
by the graphing calculator, it’s immediately available
for graphing, viewing, zooming, panning, or listing. You
can apply all the calculator’s functions to your
data set, either by hand or through simple programs.
Statistical (e.g., linear,
exponential, polynomial regressions, mean, or deviation)
and mathematical (e.g., integration, derivation) analyses
are only a few keystrokes away. All of this can be done
in the field with no other hardware except the logger
itself—a capability not found in the most expensive
data loggers available today.
The unit is battery powered
and works for more than 200 h on standard batteries while
retaining more than 20 KB of data for at least one year.
I use separate batteries for the calculator and analog
interface. Batteries make this device suitable for mobile
use, as well as eliminating ground loops, which is a common
nuisance for PC users.
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