Turtle Sense communications board v.0.44

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The Communication board v.0.44 is the controlling circuitry for Turtle Sense. It connects, via a coax cable cable, to one or more Smart Sensors. A piggybacked M2M plug-in telephony board made by Janus enables GPS and cellular communications.

Microprocessor

The microprocessor (U1) is identical to the one on the Smart Sensor 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 Janus phone board, the power supplies, and to the coax transceiver (U4 and U5). Many other I/O pins are not used, but are available on optional headers. 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.

Power supply

The Comm Board is powered from a battery pack of 3 low self discharge rechargeable NiMH AAA batteries or Eneloop equivalent. The battery pack is about 4.2V fully charged and has a capacity of 800 milliamp-hours. The battery pack is kept charged by a small solar cell mounted behind a plexiglas window at the top of the PVC enclosure. The solar cell was custom made by _____ in China, and is rated 5.8V 38 mA. A charging circuit limits and equalizes the voltage on the three AAA cells. D6 supplies a voltage reference to op amp U6d, the output of which turns on Q1 whenever the battery voltage reaches 4.2V. Q1 then shunts any additional current from the solar cell to a pair of high brightness LEDs, D3 and D4. The two LEDs serve as indicators that the batteries are fully charged. Op amp U6a compares a fraction of the solar cell voltage to a fraction of the battery voltage. When the solar cell voltage is not sufficient to charge the batteries, U6a turns off Q2, thereby saving a small drain on the batteries from R25 and D6. Op amps U6b and U6c equalizes the voltages of the three batteries by either adding or subtracting a small amount of current from the two nodes between the batteries. The battery pack powers four power supplies on the board:

  • a 3.0V linear low power voltage regulator (U3) which powers the microprocessor (U1) and related circuitry on the board.
  • a 5.0V switching voltage regulator (U2) which powers the Phone Board.
  • another 3.0V linear regulator (U7) which supplies power to the sensors via the coax cable.
  • another 3.0V linear regulator (U9) which supplies power for any auxiliary uses.

The 3.0V regulator U3 is on at all times, and its output is filtered by capacitors C5 and C6. The higher powered 5.0V regulator U2 is only turned on when the phone board is needed, in order to conserve battery power. Its output is filtered by C12, C13, C17, and C18. U2 is turned on by a positive signal (PhnPower) from the microprocessor U1. U7 is turned on by a positive signal (SnsrEn) from the microprocessor. U9 is turned on by a positive signal (AuxEn) from the microprocessor. An analog switch (U10) is used to monitor the battery voltage.