Turtle Sense Smart Sensor board v.0.46

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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.

Microprocessor

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.

Accelerometer

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.

Coax Transceiver

The Smart Sensor has a SN74LVC1G240 driver IC (U4) and a MCP6541 receiver IC (U3), identical to the ones on the Comm board. Communication and 3.0V DC power are over a single wire (RG6 75 ohm coax cable). The DC power is isolated from the AC serial communication signal by a bias T consisting of L1 and C8.

The microprocessor sends a 1 MHz signal to the input of the driver (U4). The microprocessor also sends a serial 115 kilobaud UART signal to the Enable input of the driver. Thus the output of the driver is a 1 MHz signal modulated by the UART signal. The driver output is capable of driving at least 400' of RG6 cable.

Serial UART signals are received by U3, which acts as a comparator. The non-inverting input filters the rectified incoming signal on the wire with a time constant of about 1 msec, which is a little longer than one byte of data. The non-inverting input also has a small DC bias added to keep the output of the comparator in the correct state (logic high) when no signal is being received. The inverting input of the comparator is filtered with a much shorter 2.7 usec time constant, which removes the 1 MHz carrier but retains the individual bit data.