Smart lock to secure Bluetooth LE and NFC connectivity

The smart lock is designed for both commercial and domestic applications, and once installed can be paired to the user's Bluetooth 4.0 (and later) smartphone or tablet. From the iOS and Android Xiaomi 'Mi Home' app the user can configure the lock and remote alarms, review a history of when the lock has been opened, as well as unlock the Xiaomi Mijia. From the main interface of the Mi Home app, clicking on the lock icon will instantly unlock the door using Bluetooth LE wireless connectivity.

The Nordic SoC incorporates an Arm TrustZone CryptoCell-310 cryptographic module and an AES 128-bit hardware accelerator. These features support a wide range of asymmetric, symmetric, and hashing cryptographic services for secure applications, ensuring the security of the Xiaomi Mijia unlocking process. Alternatively, the smart lock can be unlocked using Near Field Communication (NFC) via the nRF52840 SoC'son-chip NFC-A tag and a user's NFC-compatible smartphone. It also supports alternative unlocking options including fingerprint, password, and emergency key unlocking.

As the hardware management platform for all Xiaomi ecosystem smart home solutions, the Mi Home app also allows the user to connect their smart lock to other Xiaomi as well as third-party smart home devices, enabling full connectivity throughout the smart home, and the ability to perform linked events, for example close the curtains when the door is locked. The app will also send a push notification to a user in the event the lock is abnormally tampered with, or if the door has not been properly locked

The Xiaomi Mijia smart door lock employs eight AA batteries, and provides up to 18 months of battery life before replacement in standard operation, thanks in part to the ultra low power characteristics of the Nordic SoC. The nRF52840 SoC has been engineered to minimise power consumption with a fully-automatic power management system that reduces power consumption by up to 80% compared with the nRF51 Series.

Nordic's nRF52840 multiprotocol SoC is Nordic's most advanced ultra low power wireless solution. The SoC supports complex Bluetooth LE and other low-power wireless applications that were previously not possible with a single-chip solution. The SoC combines a 64MHz, 32-bit Arm Cortex M4F processor with a 2.4GHz multiprotocol radio (supporting Bluetooth 5, ANT, Thread, Zigbee, IEEE 802.15.4, and proprietary 2.4GHz RF protocol software) with 1MB Flash memory and 256kB RAM. 

A new radio architecture with on-chip PA provides features -95dBm RX sensitivity (at 1Mbps Bluetooth LE), a maximum output power of 8dBm, and a total link budget of 103dBm.The chip supports all the features of Bluetooth 5 (including 4x the range or 2x the raw data bandwidth (2Mbps)) compared with Bluetooth 4.2. 

The SoC is supplied with Nordic's S140 SoftDevice, a Bluetooth 5-certified software protocol stack for building long range and high data Bluetooth LE applications. The S140 SoftDevice offers concurrent Central, Peripheral, Broadcaster, and Observer Bluetooth LE roles, and supports high throughput and long range modes as well as advertising extensions. OpenThread and Zigbee stacks are available through Nordic's nRF5 SDK for Thread and Zigbee.

"Although other wireless protocols were an option, Bluetooth LE was more suitable as it is not only faster but also provides a better experience and satisfies our need for stability," said Tao Liu, Lumi Product Manager. "We selected the nRF52840 SoC both for its range and ultra low energy consumption. The maximum output power of 8dBm provides much greater connection range, while the low power consumption extends the battery life of the door lock.

"From a software point of view. When we require it, Nordic can also quickly provide professional technical support."