April
2005, Issue 177
Simple
USB Data Acquisition
SYSTEM
OVERVIEW
I
usually design my own boards, but for this project I
used a couple of evaluation boards to implement my minimal
USB data acquisition system. The boards are readily
available, so a hardware design isn’t required to get
up and running.
The
system is comprised of two boards, an analog temperature
sensor, and a PC running the GUI (see Figure 1). The
Keil MCB2130 evaluation board contains the new ARM-based
LPC2138 microcontroller (see Photo 1). The MCU reads
the temperature sensor’s analog output voltage via its
ADC and sends the reading via its UART. For this particular
application, I used the board’s serial port circuitry
(RS-232 transceiver and connector), expansion connector
(for hooking in the temperature sensor), and power input
connection. One of the board’s neat features is that
it’s powered from an on-board USB connector. This means
you don’t need a clunky wall wart to power the system.
You can just run another USB line to it for power. This
is a definite advantage to using USB, as long as your
board doesn’t draw more power than the USB connection
can handle.
|
(Click
here to enlarge)
|
Photo
1—I used a Keil ULINK JTAG debugger to in-circuit
debug and program the LPC2138 microcontroller. I
soldered an LM60 temperature sensor to the prototyping
area of the MCB2130 board. |
|
(Click
here to enlarge)
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Figure
1—Where does the power come from? The USB is used
for more than just communicating with the PC; it’s
also used to power both boards, which enables you
to remove those ugly black wall warts. The schematics
are posted on the Circuit Cellar ftp site. |
The
Silicon Labs CP2101 evaluation board contains the CP2101
USB-to-UART bridge chip and an RS-232 transceiver. This
allows you to plug in an RS-232 communicating device
on one side and a USB communicating device on the other.
The board and its virtual COM port software drivers
form the link between the MCB2130 board’s RS-232 port
and the PC’s USB port.
The
National Semiconductor LM60 is a simple three-pin analog
Celsius temperature sensor. It’s wired into the expansion
connector on the MCB2130 board, which connects to the
LPC2138’s ADC to read the analog voltage from the sensor.
Its output is linearly proportional to temperature (6.25
mV/°C), and it has a DC offset of 424 mV to accommodate
negative temperatures. This makes it a fairly easy sensor
to deal with in software after it’s read via the LPC2138’s
ADC.
The
PC contains the Visual Basic GUI. It reads the raw temperature
data sampled by the ADC over the virtual USB COM port,
converts it to temperature, and displays and charts
the results over time. The GUI puts the finishing touches
on the design, making it a more user-friendly and professional-looking
system. Imagine how much easier it will be to look at
graphed data instead of the streaming serial data in
a terminal emulator session.
