The SHIMMER wireless sensor platform was developed by the Intel Digital Health Group over a number of years to support a variety of internal research projects. The SHIMMER platform comprises a baseboard that provides the sensors computational, data storage, communications, and daughterboard-connection capabilities. The core functionality of SHIMMER platform is extended via a range of daughterboards that provide kinematic, physiological, and ambient sensing capabilities. This range of contact and non-sensing capabilities can be reliably used both in clinical and in home-based scenarios.
SHIMMER Baseboard Design
The core element of the baseboard is the Texas Instruments MSP430 MCU (Microcontroller Unit) , which has been widely used in wireless sensors [50-52]. The primary advantages of the MCU are its extremely low power during periods of inactivity and its proven history for medical-sensing applications. The 10Kb of RAM on the MSP430F1611 variant is the largest memory available within the processor family and offers improved buffering capability for communication applications.
The MSP430 has eight ADC channels for 12-bit A/D conversions. For the SHIMMER platform, the external ports are utilised for reading data from the XYZ accelerometer (three channels), the internal expansion connector (three channels), and the external expansion connector (two channels). To maintain the low-power usage capabilities of SHIMMER, the MSP430 ADC core is disabled when not in use and re-enabled when necessary. The baseboard has a Freescale Semiconductor 3-axis (XYZ) accelerometer (MMA7260Q). The accelerometer is suitable for low-power systems: when active the current consumption is around 500?A, and it contains a sleep mode, with current usage of 3μA. It is robust, having high shock survivability, and is suitable for high-sensitivity applications (800mV/g for 1.5g setting).
A passive tilt/vibration sensor (SQ-SEN-200 - Signal Quest) has been added to later generations of the SHIMMER baseboard. It is sensitive to both tilt (static acceleration) and vibration (dynamic acceleration). The sensor is used to trigger power state transitions. For example, when the user takes an inactive sleeping sensor out of the charging dock, the firmware will automatically switch on the necessary components (accelerometer, radio, etc.) to begin capturing and transmitting motion data.
One of the key functions of the SHIMMER board is its ability to communicate as a wireless platform. SHIMMER has the dual functionality of having both 802.15.4 and Bluetooth radio modules.
For IEEE 802.15.4-compliant wireless communication, the SHIMMER platform uses a Chipcon CC2420 radio transceiver and a gigaAnt 2.4GHz Rufa antenna.
Bluetooth Radio Module
The SHIMMER platform uses the Roving Networks RN-41 Class 2 Bluetooth module to communicate via an integrated 2.4GHz antenna. This module contains a full Version 2 Bluetooth Protocol Stack and supports the Serial Port Profile that facilitates rapid application development. The Bluetooth module is connected to the MSP430 directly via the USART1 serial connection. It can also be controlled by ASCII strings over the Bluetooth RF link.
For further functionality the SHIMMER baseboard has a Hirose Electric DF12 series* connector that allows the user to connect daughterboards to the baseboard. Currently there are seven daughterboards available for SHIMMER as in Table 1.
The external expansion (HIROSETM ST Series) allows the user to attach the board to the programming dock or multicharger.
External Hardware Integration
The SHIMMER platform provides two options for external hardware integration, such as third-party sensors. The General Purpose Expansion Module (AnEx Board) is an analog expansion module that enables two analog signals to be plugged into the SHIMMER platform. These analog signals interface with the A0 and A7 of the MSP430.
Prototype Reconfigurable board for SHIMMER (PRIMMER) is a daughterboard that enables the breakout of SHIMMER's internal connector. The purpose of the PRIMMER daughterboard is to enable rapid integration and prototyping of commercially available and existing physiological front-end and sensor circuitry. The PRIMMER daughterboard breaks out six ADC lines, power and ground prototype solder points, two general purpose input/ouput (GPIO) lines, and a Universal Asynchronous Receiver/Transmitter (UART). In addition to this, it provides a boosted power supply by way of a power-boosting circuit that takes in 3V from the SHIMMER baseboard and increases it to ±5V(Maxim MAX768EEE charge pump). The board has been utilised to interface to an EMG pre-amplifier sensor (MA-411-000, Motion Lab Systems) and to streamed EMG data at 500HZ.
GLIMMER, the SHIMMER-based display, is a low-cost, wireless display platform that enables context-aware prompting in the home for ADL monitoring, medication adherence, journaling, social messaging, and coaching applications. The GLIMMER display platform is an extension of the standard SHIMMER platform. Prompts are displayed via a 128x64x4bit 2.7-inch organic light-emitting diode (OLED) display (OSRAM Pictiva). The resolution is 128x64 with 4-bit grey for image display.
A number of peripheral components have been developed for the SHIMMER platform, and among these are the following:
- Programming dock. USB-bus-powered cradle using the FTDI FT232 UART. The dock also has a battery charger status, USB Power, and UART activity indicators.
- Multicharger. Cradle that can charge six SHIMMERs simultaneously.
- Advanced Dual UART programming board. For applications where wired serial communication or enhanced debug capabilities are desirable.
- Enclosures. The SHIMMER platform has two types of plastic enclosure. The standard housing holds the SHIMMER baseboard, lithium ion battery, and kinematic or ambient sensor daughterboards if required. The physiological enclosure is utilized with the ECG and GSR daughterboards and provides external connection points for electrodes.