Turtle Sense communications board v.0.25a

Turtle Sense v.0.25a is the Communications (Comm) Board which connects to the remote Smart Sensor Board and to a piggybacked M2M plug-in telepony board made by Janus.

Power supply
The Comm Board is powered from a battery pack of 8 rechargeable NiMH AA batteries. The battery pack is about 11V fully charged and has a capacity of 2 amp-hours. The positive battery input to the board first passes through a diode (D1) which protects the circuitry from damage if a battery pack is somehow connected backwards. The battery voltage is filtered by two capacitors, C7 and C1. The battery level is monitored by a resistor divider R11 and R12. The voltage on R12 (BattMon) is measured by an analog to digital converter, or ADC, in the microprocessor. The battery pack powers two power supplies on the board: The 3.3V regulator U3 is on at all times, and its output is filtered by capacitors C17 and C5. The higher powered 5.0V regulator U4 is only turned on when the phone board is needed, in order to conserve battery power. Its output is filtered by L1 and C12. R15 and R16 set the voltage at 5.0V. C9, C10, C11, and R17 are used for proper operation of regulator U4. U4 is turned on by positive signal (PhnPower) from the microprocessor U1.
 * a 3.3V linear low power voltage regulator (U3) which powers the microprocessor (U1) and related circuitry on the board
 * a 5.0V switching voltage regulator (U4) which powers the Phone Board.

Microprocessor
The microprocessor (U1) control all the operation of the device. Programmed in C, it manages all the circuitry by:
 * timing events
 * sending instructions to the smart sensor
 * downloading data from the smart sensor
 * monitoring the batter
 * controlling the M2M telephony board and its power supply
 * sending instructions to the telephony board to connect to the internet
 * uploading formatted data to the internet in a report
 * downloading new operating parameters from the internet
 * handling errors and other problems

We selected the Texas Instruments MSP430FR5739, here because it is extremely low power, and contains 16 K of FRAM memory. Newer versions of this chip contain more memory and will likely be used in future versions. The 3.3V power 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. Most of these ports are connected to various signals on the Phone Board and other parts of the circuitry. Most of them will be discussed later on. 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.

Plug-in telephony board connections
The telephony Board piggybacks onto the Comm Board via header sockets J3 and J4. The microprocessor communicates with the phone board with several signals, named PhnOn/Off, PwrMon, PhnReset, PhnMon, U-out, and U-in. Because the Phone Board operates at a different logic level than the Comm Board, it's necessary to shift the voltage levels of these signals to make the two boards compatible. This level shifting is done by three dual analog switches (U6, U7, and U8). Note that the signal names listed are the ones at the microprocessor. The signals have slightly different names on the Phone Board side of the analog switches.

After the Phone Board 5V power supply is turned on (as described above), the PhnOn/Off signal is used to turn on the phone circuitry. This signal is active low, meaning the processor must put out a logic 0 pulse to turn on the phone. Later, the processor puts out another logic 0 pulse to turn off the phone. The processor checks whether the Phone Board is on or off with the PwrMon signal. The processor can reset the Phone Board with a pulse on PhnReset. This pulse is also active low. The processor can check the state of the Phone Board with the PhnMon signal. The last two signals, U-out and U-in, are serial data transmission lines between the processor and phone via a standard UART protocol operating at 115,200 baud.

Smart Sensor connections
The sensor is connected to the Comm board via about 20 ft of underground Cat5e shielded cable. A short stub of the cable connects to the Comm board and to a 9-pin male Molex connector. The main length of cable connects to the sensor board and to a mating female Molex connector.

Several pins on the microprocessor are used to communicate with the Smart Sensor: SnsrRst, TX/RX ON, SensorTX, SensorRX, Interrupt, and Tamper. SnsrRst controls an analog switch (U9) which switches the 3.3V power going out to the sensor along with Ground. The output of U9 is normally connected to the 3.3V switch input. A logic high pulse on SnsrRst connects the sensor power to ground to reset the sensor. Interrupt is a signal which the sensor sends to the Comm board when it wants to communicate -- to send or receive data. Tamper is a signal which the sensor constantly keeps high. If the sensor cable is unplugged the Comm board can detect that by the logic low on the Interupt signal. A logic high level on TX/RX ON enables an RS485 Transceiver in the Comm Board (U2), which buffers the two serial data signals (SensorTX and SensorRX) going to and coming from the sensor. The TX/RX ON line is also connected via the cable to an RS485 transceiver in the smart sensor, so both transceivers turn on and off together. The microprocessor in the smart sensor monitors the TX/RX ON line to determine if communication is being requested by the Comm board.

The RS485 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. There is an identical transceiver in the sensor circuit. 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. Four resistors (R1, R2, R3, and R10) are used to provide a correct initial bias voltage on the receive inputs of both transceivers. Another resistor (R8) is a termination resistor on the receive inputs of U2 in order to prevent signal reflections travelling back to the sensor 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.

GPS and antennae
The telephony board has a built in GPS unit. There are connections on the board for two antennae, one for GSP reception and the other for Cell reception.

Known issues

 * The battery monitoring circuitry, originally designed with a much higher resistance voltage divider was found to be too high an impedance for the ADC converters in the microprocessor. Because of this, lower resistors had to be substituted in the board, causing a constant current drain of approximately 60 uA, or about 1.5 mAh a day.  While not a tremendous power loss, it is more than the power used by both microprocessors and the motion sensor.  Future versions of the board will have a switch to turn the voltage divider circuitry on and off, so the voltage will only be monitored periodically.