Issue
155 June 2003
Good
Vibrations
HANDS
ON
Motorola
offers an HC908 MCU-based evaluation module that serves
as a reference design and makes it easy to play around
with the ’33794 (see Photo 2). It’s probably well worth
the reasonable $70 price tag just to avoid the hassle
of having to homebrew with the fine-pitch, surface-mount
package.
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(Click
here to enlarge)
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Photo
2—The evaluation kit combines the M33794 with an
HC908 MCU that’s in a DIP package and socketed for
easy experimentation. |
As
you can see from the schematic in Figure 2, hooking
up the ’33794 to any MCU should be a snap. In fact,
if you ignore the automotive and glue logic extras,
all that’s really required are four or five digital
outputs (electrode select lines A to D and SHLD DIS)
and one 0- to 5-V analog input (LEVEL). One trick: if
you want to take advantage of the power-on RST output
but don’t want to be bothered with the watchdog, just
connect the WD IN pin to the ’33794’s CLK output, and
the chip itself will take care of feeding the watchdog.
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(Click
here to enlarge)
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Figure
2—Connection to an MCU is straightforward—all the
better if the MCU can take advantage of the support
functions (i.e., voltage regulator and monitor,
power-on-reset, and watchdog timer) built into the
MC33794. |
The
MCU has a small built-in monitor (source code provided)
that allows for basic ’33794 operations using an RS-232
terminal program. There are commands to select an electrode,
reference, or voltage monitor for access and turn the
shield driver on or off. The kit also comes with a Windows
GUI application that presents a graphical view of the
chip’s operation (see Photo 3).
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(Click
here to enlarge)
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Photo
3—Get your feet wet (or my finger in this case).
Look closely at the bar on the top of the EFLD.EXE
window in the background, and you’ll see how the
MC33794 detects contact with the water. |
Most
of the monitor code is command decoding, message display,
binary-ASCII hex conversion, and so on. The actual software
to drive the chip is as straightforward as you may hope
for. Set the A to D select lines to specify an electrode
(or other internal source), wait for the output to settle
(about 5 ms), and grab the 0- to 5-V analog result from
the LEVEL pin.
The
first thing I wanted to check was the basic responsiveness
of the chip. For an electrode, I used aluminum foil,
but other options include conductive foam (i.e., the
black stuff chips are shipped in to avoid electrostatic
damage), conductive tape (from burglar alarms), and
conductive paint (i.e., anything that conducts electricity).
I
quickly confirmed, as the documentation notes, that
the detection level is directly related to the area
of the electrode. Using some good old hand capacitance,
I found the threshold for proximity detection using
a square piece of foil to be about the length of a side
(i.e., 1” for a 1” × 1” electrode, 2”
for a 2” × 2”, etc.). It is no surprise that
a larger electrode increases sensitivity, but there
are limits—too large, and it starts to load the output
and reduce accuracy and dynamic range.
Within
electrode size and range constraints, the MC33794’s
sensitivity is pretty good. In fact, by slightly raising
and lowering my hand, I could see the discrete quantization
steps associated with the MCU’s 8-bit ADC. My hand-waving
experiments would have been better served with a 10-
or 12-bit ADC (or an external programmable amp).
I
connected my foil electrodes using about 0.5 m of coax.
(Note that Motorola recommends limiting the length to
1 m or so.) It was easy to see the virtue of the shield
driver feature: when I turned it off (i.e., SHLD DIS
pin), the output from the electrode was completely swamped
by cable capacitance.
