TinyOS Programming Boot Camp Part III: Hardware Tour

1
TinyOS Programming Boot Camp Part III Hardware
Tour

• David Culler, Jason Hill, Rob Szewczyk, Alec Woo
• U.C. Berkeley
• 2/9/2001

2
Hardware Tour
3
Outline

• Design overview
• Connector bus
• Sensor motherboard
• Sensor boards
• Supporting hardware
• Resources

4
Design Rationale

• Want to experiment with a wide variety of sensors
• Need a wide variety of interfaces, analog and
  digital
• Sensor platforms should be easy to design
• Want to preserve processor and communication
  module
• Major investment in SW and programming
  environment
• Want interoperability between the various types
  of sensors
• Every sensor node needs communication to be
  useful
• Need a fixed interface to match the sensor
  modules with processor and communication

5
Design Rationale

• Main board requirements
• Easy to program and debug
• Need an easy access to most signals in the system
• Need standard interfaces
• Basic self-monitoring and maintenance capability
• Ability to reprogram remotely
• Ability to control radio cell size
• Ability to monitor signal strength
• Other self monitoring added on sensor packs
• Sensor requirements
• Digital serial interfaces
• Analog sensor support
• Ability to control on/off state of individual
  sensors

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Mote Connector Schematics
7
Mote Expansion Connector

• Documented hardware interface
• Swap components on either side of the connector
  while preserving investment in sensors or main
  boards
• Sensor interfaces
• 4 lines dedicated to switching components on and
  off
• 7 analog voltage sensing lines
• 2 I2C busses
• SPI
• UART lines
• Debugging aids
• All radio-related signals RX, TX, base band,
  control signals, signal strength
• Programming interfaces
• SPI and reset signals for the main processor and
  the coprocessor
• Ground, Vcc for both analog and digital circuits
• 12 lines reserved for future use

8
Power Lines

• Need to control on/off state of individual
  sensors
• Independently switched, used as outputs
• Capability
• Sink up to 20 mA, source a bit less
• If more current is required by a sensor circuit,
  use MOSFETs
• No higher level protocol attached
• The place to implement functionality not provided
  by standard interfaces

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Analog Lines and AD Conversion

• Most sensors provide an analog interface
• Simple voltage dividers with photo resistors,
  thermistors, etc.
• Whetstone bridges, condenser microphones, etc.
• Need analog voltage sensing lines
• 10 bit ADC, ? 2 LSBgt 8 usable bits
• ? 3 mV error
• Rail-to-rail range
• Sampling rate
• Max 15.4 ksps in continuous sampling mode
• Max 4 ksps in a single sampling mode
• 8 multiplexed channels
• One dedicated to sampling BBOUT
• Sampling rate high enough for most environmental
  phenomena
• Interrupt driven, or polling version

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I2C Bus

• 2-wire serial bus clock and data
• Clock is an or of all clock generators, the
  slowest clock generator dictates the speed
• Bi-directional data line
• Higher level protocol than UART or SPI
• Defines a protocol for multiple device access, up
  to 128 devices per bus
• Defines a multiple master arbitration
• Allows for multi-byte transactions
• Speed up to 400 kbps
• Software implementation
• Soft timing constraints mean that the use of
  timers is not mandatory, use tasks instead
• Either 1 bit or 1 byte per task
• Many slave devices available
• EEPROM used for logging and other permanent
  storage
• IO extenders, ADC converters, sensors, etc.

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SPI

• 3-wire, full duplex serial bus clock, MISO, MOSI
• Connector bus defines 2 SPI busses
• Speed up to 1 mbps
• Hardware support on the main processor, software
  implementation on the coprocessor
• Low-voltage programming interface to ATMEL
  microcontrollers
• Interaction with coprocessor
• Cyclic 16 bit distributed register

12
UART

• A standard way for exchanging information with a
  PC
• Voltage conversion and connector required,
  provided by the programming board
• Available speed 2400 bps to 115 kbps
• Most reliable for communication with PC 19.2
  kbps
• Large clock rate errors at high sending rates
• 1 byte FIFOs
• Hardware doesnt buffer multiple bytes of either
  input or output
• Operate in either polling or interrupt driven
  mode
• TinyOS uses UARTs in interrupt driven mode

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Rene the Motherboard

• Main components
• Processor ATMEL AVR 90LS8535
• Reprogramming coprocessor
• Short range radio
• LEDs
• I2C EEPROM

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Rene Schematics
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AVR Processor Core

• Clock speed 4 MHz
• Memory
• 8 Kbytes of program memory (flash)
• 512 bytes of data RAM
• 512 bytes of EEPROM on chip (write 4 ms/byte)
• 32 8 bit registers
• IO capabilities
• 32 general purpose IO lines
• Some lines also serve more specialized purposes,
  e.g. UART
• 10-bit 8-channel ADC
• Connector interface means that the IO lines serve
  a more specific purposes
• Interrupts
• No external interrupts available
• No interrupt queuing

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AVR Processor Core

• Timers
• 8 bit timer/counter unused in current
  applications
• 16 bit counter one comparator used for radio
  sampling, other functions unused
• 8 bit, externally clocked timer/counter used
  for periodic sampling of sensors and sleep
  control
• More advanced timer features (input capture, PWM
  output) are available, through not explicitly
  allocated on the connector, use at your own risk
• Power consumption
• Restrictions
• No truly general purpose registers
• Limited arithmetic/logic instructions

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Programming the Processor

• Program the runtime memory image into a flash
• No loader, no dynamic linker
• Programming steps
• Extract the code and data segments from an
  executable into an SREC file
• Erase the current contents of the flash
• Reset the processor
• Download the program
• Serial download protocol
• Each download command contains the address, and
  the contents of to be stored
• 10 ms delay per byte of code
• Verify the program read the flash, match it
  against the source image
• After reset, start executing C runtime
• No ability to write the flash on the fly
• Requires a coprocessor for reprogramming

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Coprocessor Circuit

• Coprocessor ATMEL AVR AT90S2343
• 2 Kbytes of code space
• 1 MHz internal clock
• Required connections
• SPI for programming the main microcontroller
• RESET for the main controller
• I2C for accessing EEPROM storage
• Insufficient pins I2C and SPI clock lines are
  shared
• Minimal capability
• Software SPI
• Software I2C

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Radio Circuit

• RFM Monolithics TR1000 916 MHz radio
• On/off keying at 10 kbps (max. 19.2 kbps)
• Capable of 115 kbps using amplitude shift keying
• Capable of turning on in 30 us
• Low-power listening by switching on and off on a
  sub-bit granularity
• Processor interface
• RFM CNTRL 0 and 1 switch between transmit,
  receive, and sleep
• Raw, unbuffered access to transmit (RFM TX) and
  receive (RFM RX)
• Requires DC balanced signal an equal number of
  1s and 0s in the signal
• Sampling on reception and modulating on
  transmission done is software
• Too much noise in received signal to use UART for
  sampling
• too little flexibility offered by UART
• Imposes real time constraints on the system
• Power usage
• Transmit current 7 mA
• Receive current 4.5 mA
• Sleep 5 uA

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Radio Signal Strength

• Measuring the quality of reception from a node is
  useful
• Aid in multihop routing decisions
• Help with location estimation
• Signal strength measurement
• Based on base band sampling
• Demodulated and amplified analog signal, fed into
  ADC channel 0
• Raw samples further processed in TinyOS
• Empirical evaluation

Klemmer, Waterson, Whitehouse, 2000
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Radio Transmission Strength Control

• Useful in networking experiments
• Control the cell size
• Allow for power savings
• Principles of operation
• Radiated power is proportional to the square of
  the current on RFM TX pin
• Signal strength control
• Controlled through a digital potentiometer (DS
  1804), 0-50 kOhms
• Currently no observable effects on power usage
• Evaluation
• Optimal transmission at the pot setting of 10
  kOhms
• Changing the range from 45 to 90 feet in an
  indoor environment
• Changing the transmit strength has no effect on
  power usage
• Effects secondary to shape/length of the antenna

22
LEDs

• Basic mote UI and a great debugging tool
• Power consumption 6mA
• LED consumes as much power as the main
  microcontroller
• If power is a concern, turn them off!
• LED 1 and LED 2 multiplexed with the signal
  strength potentiometer control
• Use them for controlling any other DS 1804 pots,
  one at a time

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Matching SW and HW hardware.h

• Define a hardware.h for each motherboard
• Map motherboard abstractions onto physical pins
• RFM RX, TX, and control pins
• Allocate the connector pins onto the motherboard
  resources PW lines, I2C busses, UARTs, etc.
• Define a header file for each sensor board
• Assign symbolic names (e.G. PHOTO_PWR) to the
  connector signals
• Attempt to maintain a consistent convention in
  hardware design
• Photo and temperature on the same signals in both
  basic and advanced boards

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Mote Connector Caveats

• Multipurpose lines
• LED1 and LED2 are used to control the signal
  strength potentiometer. They can also be used to
  control digital pots on sensor boards
• PW3 can be used as a PWM output
• PW4 can be used as a timer input capture, useful
  for PWM inputs
• Other restrictions
• Relationship between the main processor and the
  coprocessor dictates that their SPI busses are
  permanently connected
• Shortage of pins on the coprocessor dictates that
  the SPI SCK is shared with the I2C BUS 2 CLK
• Cannot use this bus when the board is plugged
  into the programming port
• Cannot use the logging component when a board is
  set up for programming

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Basic Sensor Board

• Sensors
• Photo resistor PW1 and ADC1
• Thermistor PW2 and ADC2
• Prototyping area

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Vibes Sensor Board

• Vibes
• Photo resistor PW1 and ADC1
• Thermistor PW4, ADC6
• 2 axis accelerometer ADC2 and 3, PW2
• Digital temperature I2C Bus 1

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2xMAG Board

• Sensor
• 2 axis magnetometer optimized for vehicle
  detection
• Resources PW2, 3, and 4, ADC 6 and 7

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Programming Board

• SPI interface through the parallel port
• Programming either the main microcontroller or
  the coprocessor with a flip of the switch
• RS232 connector
• Voltage conversion
• CTS/RTS signal grounded
• Program the serial port accordingly

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Building Your Own

• Tools
• OrCAD capture and layout tools
• Complete designs
• Rene
• Basic
• Vibes
• 2xMAG
• Base board

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Resources

• Datasheets
• ATMEL 8535 datasheet
• RFM TR1000 datasheet
• 24WC256 EEPROM
• ADXL202 2 axis accelerometer
• HMC1002 2 axis magnetometer
• Schematics
• Rene motherboard
• Basic sensor board
• Vibration sensor board
• Vehicle detection board
• Base board
• Debugging board

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Exercise

• Basic sensor modification
• Add a buzzer to the basic sensor board
• Connect buzzer to Vcc and to an unused PW line.
  The buzzer should turn on when pulled PW is
  pulled low
• Write a component to interface with a buzzer
• Use a similar component as a template
• Using the previous application as a template,
  create an application which beeps when the
  average light level drops below some threshold
  (audience choice)