Sensor Node Architecture

1
Sensor Node Architecture

• The sensor nodes are the building blocks of WSNs.
• Each sensor node consists of 6 functional blocks
• CPU - the core of the node
• MEMORY SPACE to store the application program
  and data
• POWER SUPPLY provides energy for functioning
• RADIO TRANSCEIVER receives/transmits data and
  keeps the node connected to the network.
• ANTENNA
• SENSOR(s) - to get measurements from the
  environment.

2
Sensor Node Development

• Over the last 10 years many sensor nodes
  platforms developed by companies and research
  groups.
• Targeted key features
• Increased CPU processing power (e.g. useful for
  digital signal processing)
• Increased memory size
• Improved radio reliability
• Security (e.g. channel encryption and nodes
  authentication)
• Dynamic multiple operational modes (e.g.
  minimized power consumption in idle mode, but
  also possibility of faster and more powerful
  computation when needed)
• Small dimensions and compact packaging

3
Sensor Node Development
Intel Mote
MicaZ
Mica2
XYZ Node
4
Sensinode Architecture
BUS CONNECTOR
LEDs
EXTERNAL CONNECTOR
CLOCK
CPU
FLASH
RADIO
ANTENNA
5
Sensinode Architecture

• CPU MSP430F1611 by Texas Instruments
• Ultra-low power consumption 330 mA at 1 MHz
  stand-by 1.1 mA
• 16-bit RISC (Reduced Instruction Set Computer)
  architecture
• RISC small and highly-optimized set of
  instructions
• 16 bit Arithmetic Logic Unit (ALU) for shift,
  swap, operations, bit manipulation and bit test
  operations in a single cycle
• 16 integrated registers for reduced instruction
  execution time (register-to-register operation
  time 1 cycle)
• 1 active mode 5 software selectable low-power
  modes of operation
• Interrupt event to wake up the CPU from a
  low-power mode ? interrupt service program ?
  restore the low power mode
• Difference clocks active/disabled, crystal
  oscillator going/stopped
• Integrated 10 kB RAM 256 kB FLASH memory
• 12 bit A/D converter

6
CPU Registers

• Program counter
• indicates where the computer is in its
  instruction sequence (address of the next
  instruction to be executed). It is automatically
  incremented at each instruction cycle ( ?
  instructions retrieved sequentially from memory).
  Certain instructions, such as branches and
  subroutine calls and returns, interrupt this
  sequence by placing a new value in the program
  counter.
• Stack pointer
• points to the most recently referenced location
  on the stack when the stack has a size of zero,
  the stack pointer points to the origin of the
  stack
• Status register
• collection of flag bits (e.g. zero, parity,
  overflow, negative)
• Constant generator
• provides access to 6 commonly used constant
  values without requiring an additional operand

Program Counter
Stack Pointer
Status Register
Constant Generator
General-Purpose Register 1
General-Purpose Register 2
General-Purpose Register 12
7
12-bit A/D Converter

• 12 bit A/D CONVERTER
• Normally, 2 B needed to store a sample (4 bit
  unused)

X X X X 0 1 1 0
1 0 1 1 0 1 1 1

• 1 B can be enough (application-dependent)
• One must chose which bits bring more information
• 8 most significant
• ? Works well for large scale variations
• ? Lost of resolution in small scale variations
• 8 least significant
• ? High resolution in small scale variations
• ? Saturation problem

8
Sensinode Architecture

• MEMORY
• 10 kB RAM 256 kB FLASH memory integrated in the
  CPU
• RAM (Random Access Memory)
• Stored data can be accessed in any order ? any
  piece of data can be accessed in a constant time,
  regardless of its physical location and whether
  or not it is related to the previous accessed
  piece of data
• Volatile memory (no power ? information lost)
• FLASH
• Used to store the executable files of the
  application
• Slower data access speed than RAM
• High resistance to extreme conditions
  (temperature, pressure) and to kinetic shocks ?
  suitable for embedded devices
• Non-volatile memory (no power ? information kept)
• 4 Mb ( 500 kB) external serial FLASH
• Can be used to store measurements data
• Sequential data access
• Sector writing (up to 256 B) in 1.5 ms, sector
  erase (512 Kb) in 2 s, total erase (4 Mb) in 5 s.

9
Serial FLASH Operations vs. Power Consumption
power consumption
Writing
Reading
Access
time
10
Block-wise Serial FLASH Writing
The peaks of power consumption caused by
block-writings in the FLASH memory are observable
also in the collected samples as periodic noise
11
Sensinode Architecture

• RADIO CC2420 RF transceiver by Texas Instruments
• ZigBee radio, based on the IEEE 802.15.4 standard
  for low-rate wireless personal area networks
  (LR-WPANs)
• Compared to Bluetooth, simpler software and 50
  power consumption
• Frequency range 2400 2483.5 MHz
• Data rate 250 kbps (theoretical!)
• Modulation DSSS (Direct Sequence Spread
  Spectrum) and 0-QPSK with half sine pulse shaping
• Current consumption RX 18.8 mA, TX 17.4 mA
• Approximately 4 ms setup time in ON/OFF switching
• 128 B FIFO receive and transmit buffers
• Header takes about 40 B ? payload ? 80 B
• Digital RSSI (Radio Signal Strength Indicator)
• 802.15.4 MAC security
• Encryption process of transforming the
  information using an algorithm to make it
  unreadable to anyone except those possessing the
  key
• Authentication confirms that the packet comes
  from an authorized node

12
Sensinode Architecture
External Connector used to connect sensors to
the sensor node
Pin1-2
Pin1-2
Top view
Side view
13
Starting an Application

• main() function in main.c performs the
  initializations
• int main (void)
• LED_INIT() / Initializes the leds /
• if (bus_init() pdFALSE)
• debug_init(115200) / Initializes the debug
  window /
• stack_init() / Initializes NanoStack /
• / Creates the tasks/
• xTaskCreate(My_Task,MY_TASK",256,NULL,(tskIDLE_P
  RIORITY1),NULL)
• vTaskStartScheduler() / Starts the scheduler
  /
• return 0

14
Application Task

• The execution continues in the created task (or
  in the task with the highest priority, if
  multiple tasks were created)
• The application task runs in an endless for loop
• When the task is sleeping, the scheduler can
  handle other tasks
• Sleeping occurs when blocking on a queue or
  semaphore, or by calling vTaskDelay() or
  vTaskDelayUntil()

15
NanoStack Radio Communications

• NanoStack radio communications are specified in
  socket.c
• buffer.h needed to handle the content of the
  transmitted packets
• socket_t Radio_Socket 0 / Socket declaration
  /
• buffer_t Packet_Buffer / Buffer declaration /
• define MY_PORT 10 / Ports definition /
• / Address declaration /
• sockaddr_t BroadcastAdd
• ADDR_802_15_4_PAN_LONG / Address type /,
• 0xFF, 0xFF, 0xFF, 0xFF,
• 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF , /
  Destination address /
• MY_PORT / Destination port /
• RT_Socket socket(MODULE_CUDP,0) / Socket
  creation /
• socket_bind(Radio_Socket, BroadcastAdd) /
  Socket bind /

16
NanoStack Radio Communications

• Packet_Buffer socket_buffer_get(Radio_Socket)
  / Allocate a buffer to the socket /
• socket_sendto(Radio_Socket,BroadcastAdd,Packet_Bu
  ffer) / Packet transmission /
• Receive_Buffer socket_read(Radio_Socket,2000)
  / Packet reception /
• Once received data are handled, buffer must be
  freed
• socket_buffer_free(Receive_Buffer) / Free the
  buffer /
• Receive_Buffer 0 / Set the pointer to 0 /

17
NanoStack Radio Communications

• Other useful radio functions
• rf_power_set (new_power) / Set transmitting
  power ( 0,100) /
• rf_channel_set (channel) / Set the radio
  channel (11,26) /
• rf_rx_disable () / Turn off the radio /
• rf_rx_enable () / Turn on the radio /
• rf_set_address (address) / Set MAC address of
  the node /
• rf_analyze_rssi () / Get the RSSI (in dBm) /