PNI MicroMag - Manual (Page 19)

MicroMag User Manual – March 2010 Page 19 of 23
5 Operation
A single 8-bit command from the host system configures and initiates a sensor
measurement. Only one sensor can be measured at a time. Each magneto-inductive
sensor operates in an individual oscillator circuit composed of an external bias resistor along
with digital gates and a comparator internal to the PNI ASIC. (See Figure 4-1.)
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 made 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.
For a more detailed discussion on the underlying operation of the MicroMag module
please refer to the PNI ASIC User Manual ( http://www.pnicorp.com/support/manuals ) and/or
PNI’s Magneto-Inductive Technology Overview ( http://www.pnicorp.com/support/articles ).
Data flow to and from the MicroMag is through a hardware-handshaking, synchronous
serial interface adhering to the SPI bus protocol. Section 4.3 reviews the various interface
lines.
5.1 SPI Port Timing
Figure 5-1 graphically shows the timing sequence for the MicroMag. The clock
polarity used with the MicroMag must be normally LOW (cpol = 0).