June
2006, Issue 191
Earth
Field Magnetometer
Cypress
PSoC High Integration Challenge 2004 Winner
MAIN
ROUTINE
The
main routine is shown in Figure 4. After initialization,
the system enters Sleep mode until you press a push
button, a transition occurs on the serial receive data,
or the sleep timer expires. Once each minute, the analog
hardware is powered on and a new measurement is stored
in the EEPROM. It’s arranged as a 16-KB circular buffer,
which can store about 10 days of data. I save bits 11:4
of the 14-bit measurement, which gives a resolution
of 4 nT and a 1,024-nT range. Values outside this range
will wrap around.
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(Click
here to enlarge)
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Figure
4—The main routine waits for a button press or incoming
serial activity while taking measurements at regular
intervals. |
The
sleep timer isn’t accurate enough to time the samples,
so I calibrate it against the main oscillator by computing
the number of sleep timeouts needed for 1 min. Because
the low-speed oscillator can drift over time, this calibration
is repeated once per hour. The purpose of the magnetometer
is to examine trends in the magnetic field variations
rather than log the exact times of occurrences, so precision
isn’t needed.
