System Architecture of Sensor Network Processors Alan Pilecki

Outline What is a Sensor Network? What are the requirements of a processor in a Sensor Network? What design decisions have been made based on these requirements? What is the current state of the art in this field? What does the future hold?

Sensor Networks (1) Designed for specialized applications including: Habitat monitoring Implanted medical devices (e.g. heart monitor) Military detection systems

Sensor Networks (2) Hardware Specialized processors/coprocessors Radio Transceiver Sensor (e.g. audio, infrared, temperature, vibration, etc.)

Requirements 1. Low power consumption = Long life span 2. Self-powered Battery Power harvested from environment (e.g. solar power, vibration, electro-magnetic radiation) 3. Small in size 4. Very reliable / no maintenance

Low Power Consumption This is the main driving force for the design of a sensor processor. Ultimate goal is for a deployed sensor node to provide continuous sensing for years without being touched. Decisions made to save power may result in slower CCT, acceptable as long as specialized applications need is met.

Common characteristics (1) Event Driven Event execution/queue implemented entirely in HW, eliminating need for OS. Accelerates processing and dispatching of events.

Common Characteristics (2) Modularized specialized hardware. Accelerate common operations. Easily expandable. Allows for more power saving options. Majority of HW is put to sleep when not actively sensing. Quick wakeup time.

SNAP/LE (1) Sensor Network Asynchronous Processor / Low Energy Asynchronous processor No clock for sequencing in any component. Synchronization done with a handshaking protocol between hardware components. Results in extra hardware (Async overhead). Every signal must be without glitches or switching hazards (no clock to help recover). Results in much lower power consumption.

SNAP/LE (2) Processor Core Event queue Instruction fetch Lookup table to map events to their specific handlers. ISA As simple as possible Main purpose is to execute event handlers.

SNAP/LE (3) Time coprocessor 3 self-decrementing timers. Used to wake up the system to check the state of the sensor environment. Message coprocessor Interface between processor core and the external HW (radio and sensor).

SNAP/LE (4)

Features of other Sensor Processors (1) Hardware power leakage Older technology is used in a lot of cases to minimize inactive power leakage. Memory power saving: SRAM divided into banks of 256 bytes. Unused portions of memory are gated. Reduces active and leakage power. Resulting in over a 98% power reduction.

Features of other Sensor Processors (2) Event priority levels Two inputs to the event scheduler. High priority events can pre-empt low priority events. General Purpose Processor Used for uncommon operations. Gives programmer more flexibility. Consumes much more power than specialized processors.

Another example

What does the future hold? Increase lifespan of sensors Improved battery life (slow developing and expensive) Harvesting power from the environment Sensors that can run for long periods of time by recharging their power source from their surroundings. Will open up the use of sensors to other fields/applications.

Questions???