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 size4. 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???