View source for Imote2 560Hz Accelerometer Sampling

The Imote2 by default samples at 280Hz.  To enable 560Hz sampling, you need to modify the following file:

<pre>
$TOSROOT/contrib/imote2/tos/sensorboards/BasicSensorboardAccelDataM.nc
</pre>

Line 20 defines SPEED whose value corresponds to bits DF1 and DF0 of CTRL_REG1, which is defined on page 14 of the accelerometer's [http://www.st.com/stonline/products/literature/od/10175/lis3l02dq.pdf user's manual]. Specifically, 0 corresponds to 280 Hz, 1 corresponds to 560 Hz, etc. Change this value to 1 to get the sampling frequency of ~560Hz.

* 0 -> 280Hz
* 1 -> 560Hz
* 2 -> 1120Hz
* 3 -> Not Valid
* 4 -> 4480Hz (Appears to be successful, but not valid on oscilloscope)

== Measuring the Accelerometer's Sampling Frequency ==
The actual sampling frequency of the accelerometer on the Imote2 depends on the sensor board's crystal.  This frequency may differ from the optimal by up to 10% in either direction.  This section describes how to determine the actual sampling frequency using an oscilloscope.

According to page 2 of the [[Media:Intel.ITS400.DataSheet.pdf]], the accelerometer is by default connected to SSP1 and generates an interrupt on GPIO96/FF_RXD.  Page 5 specifies that the FF_RXD is mapped to pin B1 of the large "B" connector.  A diagram of the ports is shown in page 4 and is duplicated below for your convenience:

[[Image:ITS400pinout.png|center|thumb|500px|The various connectors on the ITS400 sensor board.]]

When looking at the top of the ITS400 sensor board with the analog sensor connectors on the left, port B1 is the top-right most pin of the large 31-pin connector.  Use the Oscilloscope's probe to measure the voltage of this pin.  A 580Hz sampling rate corresponds to a sampling period of 1.724ms.  Thus, set the oscilloscope to be 1ms per division along the horizontal axis.  The Y-Axis should be set to 1V per division.

Use an application like TestSensorboard to access the accelerometer.  Instructions for running this application  are given [[Testing_the_Imote2_Sensor_Board|here]].  Once the accelerometer is being accessed, the oscilloscope should start displaying a square wave form showing each time the accelerometer signals RDY.  Here is an example screen shot taken from a Tektronix TDS 2004B oscilloscope:

[[image:Oscope527Hz.png|center|thumb|320px|The Oscilloscope measuring the voltage of pin B1.  The cursors show that the period is 7.760ms, which is 568.2Hz.]]

It shows that the actual sampling frequency is 568.2Hz.