PNI MicroMag - Manual (Page 23)

MicroMag User Manual – March 2010 Page 23 of 23
5.4 Making a Measurement
To make a sensor measurement, a command byte is sent to the MicroMag through
the SPI port specifying the sensor (axis) to be measured and the “Period Select”. The
Period Select defines the number of oscillation cycles (periods) to be measured in both
the forward and reverse bias directions. A sensor measurement consists of measuring
the time required to complete the host-specified number of oscillation cycles (set by the
Period Select) in both the forward and reverse bias directions. The measurement is
ma
de with the PNI ASIC’s internal high speed clock, which runs at nominally 2 MHz.
The MicroMag returns the difference between the two measurement times represented
as a number in a 16- bit 2’s complement format (range: -32768 to 32767), and this
number is directly proportional to the strength of the local magnetic field in the direction
of the specified axis. Note that the greater the Period Select value, the higher the
resolution of the measurement and the longer the sample time.
The sequence and timing are discussed in Section 5.1 and the command byte
syntax is discussed in Section 5.2.
The steps to make a sensor measurement are as follows:
1. SSNOT pin is set LOW. (This enables communication with the master device.)
2. RESET pin is set HIGH, then LOW. This will reset the PNI ASIC. It is necessary
to reset the MicroMag before each measurement.
3. A command word is clocked into the MicroMag on the MOSI pin. Once 8 bits
have clocked in, the MicroMag will execute the command (take a measurement).
4. A measurement consists of forward biasing the sensor and making a period
count; then reverse biasing the sensor and counting again; and then taking the
difference between the two directions.
5. At the end of the measurement, the DRDY line is set HIGH, indicating data is
ready. The data is clocked out on the MISO pin with the next 16 clock cycles.
If another measurement is to be made, start at Step 2. The next command can be
sent after resetting. In this case, SSNOT should be kept LOW. If the MicroMag will not
immediately be used again and there are other devices on the SPI bus, set SSNOT to
HIGH to free up the SPI bus.