PNI RM3000 Evaluation Board - Manual (Page 13)

RM3000 Evaluation Board User Manual – June 2011 Page 13 of 19
5 Operation
The basic functions to be performed when operating the RM3000 Evaluation Board 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.
The RM3000 Evaluation Board 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 RM3000
Evaluation Board 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
module to write the measured values out on the MISO line for up to 3 sensors. For two or three
axis measurements, 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 512
D
, 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 cycle counts. Lowering the cycle count value reduces acquisition time, which increases
maximum achievable sample rate or, with a fixed sample rate, decreases power consumption.
See Figure 3-1, Figure 3-2, and Figure 3-3 to estimate the appropriate cycle count value for
your application. Once the Cycle Count Registers are set, they do not need to be repopulated
unless the user wants to change the values or the system is powered down (in which case the
default values would populate the register fields when powered up again).