Turtle Sense communications board v.0.25a

Circuit Description for Turtle Sense v.0.25a

Turtle Sense v.0.25a is the Comm (communication) Board which connects to the remote Smart Sensor Board and to the piggybacked Phone Board. The Comm Board is powered from a separate battery pack of 8 AA rechargeable Eneloop 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 circuit 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: (1) a 3.3V linear low power voltage regulator (U3) which powers the microprocessor (U1) and related circuitry on the board; and (2) a 5.0V switching voltage regulator (U4) which powers the Phone 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.

The microprocessor (U1) does lots of stuff inside its tiny little chip. Its program is very, very complicated, and was written by Sam. You don't want to know about it. Really. But if you insist, you can read all about the microprocessor, which is a Texas Instruments MSP430FR5739, here. 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 many 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.

The Phone 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.