Issue
155 June 2003
E-Field
Evalulation Module
DANGER,
WILL ROBINSON
Now
that you know what MC33794 devices do, let’s put a couple
of them to work. The electronic device sporting an MC33794
in Photo 2 is Motorola’s e-field evaluation module (EVM).
From left to right, the main players are an MC33794,
an MC68HC908GR8CP microcontroller, and an ever-present
Sipex SP232ACP. If you take a closer look, you’ll see
that some parts seem to be missing. They’re actually
there but on the opposite side of the board in SMT form.
I used the EVM as an EFID trainer and baseline hardware
development tool. While I was waiting for my gaggle
of MC33794 devices to arrive, I removed the MC33794
from the EVM you see in Photo 2 to verify its footprint
against my CAD drawing. In that removal and resolder
process, I fried some capacitors and the 32-kHz crystal
that clocks the microcontroller. After some minor through
hole-to-SMT electronic surgery, I got my EVM back online.
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(Click
here to enlarge)
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Photo
2—Besides some new SMT components and a set of header
pins, this is an out-of-the-box Motorola e-field
evaluation module. I put this little board through
the ringer during the research and development phase
of this column. All of the basic building blocks
found in the EVM are designed into my Z8-based version,
called the Z8 E-Field EVM. |
As
soon as I had positive confirmation of my MC33794 PCB
footprint data and a born-again EVM, I proceeded to
place various capacitors on the EVM’s Ref A and Ref
B pads to get a feel for the voltages produced by different
capacitance values. The EVM features a useful software
tool that monitors and displays all of the sensor values
(thanks to the Sipex SP232ACP) in real time on a serially
attached PC. The Motorola e-field program is shown running
in Photo 3.
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(Click
here to enlarge)
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Photo
3—Electrode E1 is attached to one of my aluminum
foil sensors and has my HP16C Computer Science calculator
in its electrical field. Ref A is loaded with a
10-pF capacitor. The pads of Ref B are supporting
a 100-pF capacitor. The numbers are 8-bit ADC clicks
from the Motorola MC68HC908GR8CP with a 5-V reference. |
The
nice color display you see in Photo 3 is attributed
to the resident firmware embedded in the MC68HC-908GR8CP
microcontroller. I used my Serial Test Async RS-232
sniffer program to look at the datastream flowing between
my PC serial port and the EVM. Using a terminal emulator
like Tera Term Pro or HyperTerminal to select an electrode
manually, I entered S 1 (i.e., S space 1) for electrode
1 and then entered an “X” to perform the ADC read of
the MC33794 LEVEL pin. The resolved voltage of the selected
electrode was then displayed in the terminal window.
The e-field PC program simply issues the same commands
issued in the terminal session to the microcontroller
on the EVM and displays the electrode voltages in a
graphical format. Some of this trapped command data
is shown in the Serial Test Async capture session you
see in Photo 4.
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here to enlarge)
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Photo
4—The e-field program is issuing commands in the
white areas of the capture. The Motorola e-field
EVM is prompting and responding in the shaded areas
of the capture. VPWR in Photo 3 is associated with
PWR_IN_MON in this screen shot. In the fifth from
top shaded ASCII area, you can see that the value
of hexadecimal 87 was returned for PWR_IN_MON, which
equates to the 135 decimal value you see in the
Photo 3 graphic for VPWR. |
