# I3 Mote: An Open Development Platform for the Intelligent Industrial Internet
## 4 Platform Hardware
### Power Management  
The I3 Mote integrates a power management unit (see Figure 4) capable of collecting energy from thermoelectric, solar and other harvesting sources. The energy can be stored into a large capacitor reservoir (through an external connector) or a secondary cell (i.e., a rechargeable battery).  
Once there is enough charge stored in that bank the system can be powered from the reservoir. Alternatively, if sufficient charge has not been collected, a backup non-rechargeable battery (e.g., AA or AAA) can provide power to the system. The power management device gives an indication of the actively used power source (harvesting cells or battery) to the microcontroller unit (MCU) by using a general purpose input/output (GPIO) pin.  
There are two main voltages in the system: 3 V and 5 V. The 3 V (VCC_MAIN) source is used for MCUs, sensors, switches, etc. and the 5 V is only used for external logic. The 3 V is driven by two DC/DC stages. The first stage relies on a boost/bypass DC/DC converter (TPS610981) that is capable of raising the voltage to 3.3 V from voltages as low as 0.7 V. Remarkably, this allows to power the system from a single alkaline cell. The second stage is driven by an ultra-low power DC/DC converter (TPS62737) which generates the 3 V voltage for the main power domain. This buck converter accepts input voltages up to 5.5 V, which allows the use of 5 V power supplies for the I3 Mote (e.g., USB). A P-FET based device (CSD75208) protects the battery from this 5 V source as well as power or-ring, i.e., if the USB power is connected then the battery power is not drained. From the power management unit, it is worth mentioning the following components:  
* _Energy Harvesting Manager IC_: The BQ25505 regulates the energy provided by the energy harvesting cells. The BQ25505 is specifically designed to efficiently extract microwatt to milliwatt of power generated from high-impedance sources, as solar cells, using maximum power point tracking (MPPT). The harvested power is stored in a ceramic super-capacitor reservoir or a rechargeable battery. The BQ25505 also manages a backup battery, which is used when there is not enough ambient energy to power the sensor node. Furthermore, by changing the values of three external resistors, developers can set the voltage levels of the energy reservoir that allow the BQ25505 to properly connect and disconnect the two power sources to the load. The state of the battery is monitored via the VBAT_OK line. By means of this line, the BQ25505 indicates to the system processors if the rechargeable battery is between the programmed load levels or, on the contrary, the power manager has switched to operate from the backup battery. In this way,  
Figure 4: I3 Mote power distribution.  
the application can make the appropriate decisions accordingly. Developers can also program the MPPT sampling network to optimize the input power provided by the solar cell.
* _Primary and Secondary Batteries:_ An embedded CR2032 cell battery can be used as a backup battery for the continual sensing operation. This battery type was chosen due to its ubiquity and its small size. The on-board battery holder can be used both as backup non-rechargeable battery (primary) and as the rechargeable battery (secondary). In addition, the I3 Mote incorporates standard interfaces for connecting larger external batteries, both primary and non-rechargeable.
* _Power Switches Control:_ The I3 Mote provides software control to independently switch on/off different blocks of the circuit. For this purpose, the I3 Mote includes a TS5A3160 single-pole double-throw (SPDT) analog switch. The switch enables the SSI 3 V output. Additionally, there is a specific boost converter (TPS61222) to enable 5 V at the SSI. Both 3 V and 5 V sources are designed to provide a clean power to external components (e.g., sensors).