Electronics and radio
Minimizing energy consumption was one of the major targets of WaDeD, and it was essentially what guided us in choosing the electronic components. The task was harder than if we did not have to care about energy, but the result is rewarding. In this page we explain our choices and the challeges they imposed.
For the radio, we chose the SX1231 transciver.
The SX1231 works with sub 1 GHz frequencies, allowing us to send our messages with lower energy consumption and greater distance than what popular over 1 GHz frequencies technologies allow, such as Wi-Fi, Bluetooth and ZigBee. Working with sub 1 GHz frequencies was one of the most important challenges of the project, since they are poorly explored in the market. As a direct consequence, open solutions virtually do not exist, proprietary options were very expensive and developing our own radio solution would be with very few support or guidance from anyone. In particular, price conditions would make the production of 10 WaDeD unthinkable in the conditions of our project if we used something else than a simple transceiver, which was a potential problem if case we needed to make adaptations.
Choosing to use the SX forced us to develop all the radio stack by ourselves, without any model on which to base ourselves for the implementation. This mean that besides developing a complex and innovative network, we also developed one of the few sub 1 GHz technologies with a working radio stack that are available nowadays. In a few weeks. Without having any radio knowledge prior to the course.
Micro Controller Unit
For the MCU, we used the STM32L151CB.
STM32L is STMicroelectronics’ low consumption MCU family. They were the perfect MCU for our goal, allying low energy consumption with high processor performances. Since its processor is an ARM Cortex M3, we were already familiar with its development environment, which really sped up our development process.
To store network data, we used the FM25V20-GTR F-RAM.
Ferroelectric RAM (or F-RAM) is a random-access memory that uses ferroelectric properties to achieve non-volatility. It allows us to turn the F-RAM down when not in use and therefore saving energy, and to make the MCU sleep in its deepest mode without losing data that we need to keep over time.
To provide a user interface, we used a Micro-USB plug.
We needed a way for the device to get energy and to exchange data. USB is pretty standard, which allows WaDeD to be compatible with a large range of devices. Note that we were deceived on that point, since it appeared that USB on-the-go was in general not possible on Android devices.