June
2006, Issue 191
Earth
Field Magnetometer
Cypress
PSoC High Integration Challenge 2004 Winner
HARDWARE
The
CY8C22213 microcontroller at the heart of the magnetometer
controls the feedback loop from the fluxgate sensor
to the Helmholtz coil. It also interfaces with an external
16K × 8 I2C EEPROM, a 2 × 16 LCD, an RS-232 serial port,
and a five-pin ISSP connector.
Figure
1 shows the system as well as the CY8C22213’s internal
module configuration. All of its three analog and four
digital blocks are used along with the I2C
hardware. Figures 2 and 3 (p. 67) show the digital and
analog sections.
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(Click
here to enlarge)
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Figure
1—The CY8C22213 microcontroller is at the heart
of the system. The fluxgate signal conditioning
was performed outside the CY8C22213 to minimize
noise and offset. |
|
(Click
here to enalrge)
|
Figure
2—The digital circuitry is straightforward. The
LCD shares some pins with the ISSP port, so it must
be unplugged when programming the CY8C22213 microcontroller
to avoid interference. |
|
(Click
here to enlarge)
|
Figure
3—The driver supplies a square wave to the sensor
as well as a sync signal to enable the integrator
input only when the drive coil is saturated. The
Helmholtz coil is fed with an adjustable offset
to compensate for the local static field (VR2),
a programmable offset from the PSoC (R22), and the
sensor feedback signal (R23). R22 and R23 should
be ratio-matched to within 0.1%. |
The
RS-232 interface uses a few transistors for level conversion
rather than an interface IC. It’s powered entirely by
the serial port, which lets it wake up the system on
incoming serial data without using any standby power.
The interface is half duplex, which was fine for this
application. The I2C port uses the CY8C22213’s
internal pull-up resistors, which work only for slow-speed
(100-kHz) bus transfers.[3]
The
system is powered by a 9-V battery, which drives a 5-V
regulator feeding the CY8C22213 and an 8-V regulator
powering the fluxgate drive coil. An output pin switches
power to the LCD. Another output pin controls the analog
circuits. The system needs about 80 mA while taking
a measurement, 1.5 mA when driving the LCD, and less
than 0.25 mA when asleep.
