Turtle Sense Smart Sensor board v.0.46

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Revision as of 18:29, 30 June 2016 by Dave (talk | contribs) (Accelerometer: Edit for different U numbers and new power source.)
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Smart Sensor v.0.46 is the sensor board which connects to the main Communications Board. The Smart Sensor contains a microprocessor, an accelerometer sensor IC, and a Transceiver IC.


The microprocessor (U1) is identical to the one on the Comm Board. It controls all the operation of the device. Programmed in C, it manages all the circuitry by:

  • timing events
  • controlling, reading and recording data from the accelerometer IC
  • uploading data to the Comm Board

We selected theTexas Instruments MSP430FR5949 because it is extremely low power and contains 64 K of FRAM memory. The 3.0V power from the Comm Board comes into U1 on AVCC and DVCC. The power ground connections are on AVSS and DVSS. There are three 8 bit I/O ports in the chip, labelled P1.0-P1.7, P2.0-P2.7, and P3.0-P3.7, plus three other I/O pins labelled P4.0, P4.1, and P4.4. Some of these ports are connected to the accelerometer (U2), the moisture sensing circuit (C12-13 plus other parts), and to the coax transceiver (U3 and U4). Many other I/O pins are not used, but are available on an optional header.. There are two pins connected to an external crystal (X1). X1, along with C1 an C2, provide the chip with a precise 32.768 KHz clock which is used for the timing functions in the program. The chip also has other internal higher frequency clocks. Another set of pins on the chip, labelled J.0-J.3, -RST, and Test, are used for programming the chip. These pins are connected to a PC via the JTAG connector J2. Since the Smart Sensor is cast in polyurethane resin, after it is cast it cannot be programmed again. However, many of the operating parameters can later be changed by downloading them from the Comm Board.


The accelerometer sensor IC (U2) is an Analog Devices ADXL362. The accelerometer measures the forces (including gravity) acting on it in 3 axes, and also temperature. The IC is extremely low powered and low noise. The microprocessor controls an analog switch (U4) which switches the 3.3V power going to the accelerometer. 3.0V power is supplied to the chip from the microprocessor I/O pin P2.7. The accelerometer has two power inputs, labeled Vio and Vs. Vio is the power for the digital circuitry, and Vs powers the low level analog circuitry. Vs is filtered from any digital noise on Vio by R7 and C7. The accelerometer communicates with the microprocessor via a standard 4-wire SPI interface. These four signals are labeled /CS, CLK, MOSI, and MISO. In addition the accelerometer has two interrupt lines (INT1 and INT2) going to the microprocessor.


The Smart Sensor has an ADM 3483 RS485 transceiver IC (U2) identical to the one on the Comm Board. The two transceivers send and receive data over the sensor cable as differential twisted pairs. That is, there are two wires for transmitting data (labeled Y and Z on the transceiver), and two wires for receiving data (labeled A and B on the transceiver). The reason for this is to improve noise immunity on the cable and to allow faster data rates. Note that the data pairs are crossed between the two transceivers - that is, the send pair on one transceiver is connected to the receive pair on the other transceiver. The communication uses standard RS485 UART protocol at 115,200 baud. A resistor (R8) is a termination resistor on the receive inputs of U2 in order to prevent signal reflections travelling back to the Comm Board transceiver outputs. The RS485 Transceivers in the circuitry are capable of driving a signal through several hundred feet of cable. However, the response timing in the program may need to be adjusted if long lengths of cable are used.