PNI RM3000 Sensor Suites - Manual (Page 29)

RM3000 & RM2000 Sensor Suite User Manual r08 Page 29 of 41
5 3D MagIC Operation
– Standard Mode
Note: This section discusses how to operate the 3D MagIC in Standard Mode. For a description of
operation in Legacy Mode, see Section 6. The 3D MagIC operates in Standard Mode when pin #12 is
held LOW (grounded to DVSS).
The basic functions to be performed when operating the 3D MagIC are:
Setting the values in the Cycle Count Registers, and
Taking sensor measurements.
The user should first establish the number of cycle counts to be measured for each sensor by
writing to the Cycle Count Registers. This is followed by sending a command or series of
commands to make the sensor measurements. Assuming the user will use the same number of
cycle counts for subsequent measurements, it is not necessary to rewrite to the Cycle Count
Registers for subsequent sensor measurements.
In Standard Mode, the 3D MagIC provides two methods to take sensor measurements, which are
discussed later in this section:
Single-axis measurement (SAM), and
Multi-axis measurement (MAM).
The SAM Command Byte initiates a measurement for one sensor and sets up the 3D MagIC to
write the measured values out on the MISO line. The MAM Command Byte initiates a sensor
measurement for up to 3 sensors, and a later Command Byte sets up the 3D MagIC to write the
measured values out on the MISO line for up to 3 sensors. For two or three axis systems,
normally it is more efficient to operate using the MAM Command Byte.
5.1 Cycle Count Registers
Prior to sending a command to take a sensor measurement, it is necessary to write values to
the Cycle Count Registers. (The default value for the Cycle Count Registers is 512D, but
this was chosen for legacy reasons and is an inefficient value otherwise.) The Cycle Count
Registers establish the number of sensor oscillation cycles that will be counted for each
sensor in both the forward and reverse bias directions during a measurement sequence.
Increasing the cycle count value increases measurement resolution, but system noise limits
the useable resolution such that the maximum efficient cycle count value generally is around
200-300 cycle counts. Lowering the cycle count value reduces acquisition time, which
increases maximum achievable sample rate or, with a fixed sample rate, decreases power