Each layer is printed on a 53 x 53 mm PCB and it is populated by 4 through hole 14-pins connection ports one for each side. Each port forwards signals to the top and to the bottom making it possible to create a stack of several layers.
The MCU layer
The MCU layer contains the MSP430F5418A micro-controller unit (MCU), the 512K serial eeprom, the 48-bit Extended Unique Identifier (EUI-48), and the Real-Time clock calendar.
The micro-controller unit has an optimized architecture and exhibits a large set of low-power inactive modes (ranging from ≈0.1µW to ≈73µW) conceived to achieve extended battery life in portable measurement applications and to enable the implementation of autonomous WSNs harvesting supply power from the environment. Nevertheless, the MCU is a powerful 16-bit RISC CPU running at up to 25MHz with a built-in digitally controlled oscillator (DCO) which allows wake-up from low-power to active modes in less than 5 μs.
The Radio and Sense layer
The radio layer hosts a Texas Instruments CC2520 RF transceiver, a second-generation ZigBee/IEEE 802.15.4 RF transceiver working in the 2.4 GHz band. CC2520 RF provides several integrated features, including: hardware support for frame handling, data buffering, burst transmissions, data encryption, data authentication, clear channel assessment, link quality indication, frame filtering, standard RX mode, and low-current RX mode. All these built-in features significantly reduce the MCU computational load and power consumption.
Temperature, luminosity, and pressure sensors are pre-installed on this layer to facilitate simple applications development.
The Programmer layer
The programmer and debugger layer is a low-cost solution to fully programs and debug the VirtualSense node. In fact, it allows MSP430 programming, througth the built-in USB port (in BSL mode), and RS323 communication without need to buy a dedicated programmer hardware or software. In the same time, it allows JTAG programming and debugging, through a dedicated port, using a standard Texas Instruments MSP-FET430UIF programmer.
Finally, the programmer layer acts also as a power supply converting the USB 5V to a 3.3V to powering VirtualSense.
The Power Supply layer
The Power Supply layer hosts 2 AAA batteries needed to powering the VirtualSense mote. It is designed to easily test and deploy a small in-lab network. It is also equipped with an on-off switch and with a protection fuse.
The Ultrasonic Wake-up and
Distance measurement layer
The Ultrasonic Wake-up and Distance measurement layer gives to VirtualSense the capability of triggering the wake-up of a node over the air. This layer supports a simple out-of-band addressing scheme to enable the selective wake-up of a target node. In addition, it makes it possible to exploit the propagation speed of ultrasonic signals to perform distance measurements.