Design Realization lecture 19

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Design Realization lecture 19

• John Canny
• 10/28/03

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Last time

• Sensors

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This time

• Real-time programming

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Threads and Processes

• A thread is a sequence of program steps that may
  be executed concurrently (with other threads).
• Threads contrast with processes, which run in
  different address spaces. Threads share a common
  address space.

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Threads and Processes

• Inter-process communication requires file,
  message or socket communication.
• Threads can communicate simply via shared
  variables.

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Threads in real-time code

• Threads are almost always a good idea in
  real-time code.
• They provide a clean way to deal with outside
  events happening at different rates.
• Switching between threads can be very fast,
  supporting fast event streams.

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Threads and interrupts

• On a single-processor machine, the processor
  executes all the threads (not true concurrency).
• Changes from one thread to another (context
  switches) are triggered by hardware interrupts,
  and by software (e.g. return instruction).
• Typical interrupt triggers
• Serial port send/receive buffer full (or empty).
• Digital I/O pin change
• A/D or D/A conversion complete
• Timer interrupt

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Interrupts and masks

• Interrupt masks specify what the processor should
  pay attention to. You set these first then
  execute an enable interrupts instruction.
• You can also disable interrupts while executing a
  critical section of code, e.g. processing another
  interrupt. But make sure to re-enable them fast
  enough to catch the fastest event stream.

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Interrupts and state saving

• Interrupts on small machines dont save much
  state. Perhaps only the program counter and a
  status byte.
• Youre responsible for saving any variables that
  are shared by the original program and the
  interrupt routine.
• You should then restore these values before you
  return from the interrupt.
• But check whether the compiler takes care of
  this

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Interrupts vs. polling

• Sometimes an event doesnt trigger an interrupt,
  or it may not be desirable to allow a certain
  kind of interrupt.
• In such cases the code can explicitly test the
  condition periodically.
• This is called polling.

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Inter-thread communication

• Usually no direct software support implement
  using semaphores in variables
• Main thread
• DataReady 0
• // Set some data here
• DataReady 1
• Other thread
• While (! DataReady)
• // Read new data
• // Set DataReady 0, or set another semaphore

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Example

• Serial data to r/c control servo PIC program.
• Receives time-stamped values for two servos over
  a serial port.
• Variable network delays, so data must be
  delay-adjusted.
• Output is r/c pulse data, a kind of PWM.

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r/c servo control signals

• Each servo is controlled by a pulse whose width
  varies from 1-2 ms and occurs every 20 ms.
• To get 8-bit (256 levels) resolution in the pulse
  width requires a temporal resolution of 1 ms/256
  4 ?s.

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r/c control example

• Real-time needs serial port data receive and r/c
  pulse output.
• Delay correction requires a moving average filter
  which is math intensive.
• Implement with 4 threads
• Two timer threads
• One for clock updates (to resync the data)
• One for PWM updates
• One polling thread for serial input
• One main thread for math
• Communication with semaphores

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Larger systems

• eCos an open-source operating system for small
  (PDA-sized) devices.
• Architectures PowerPC, ARM, IA32, H8, M68K
• Component-based and configurable only needed
  modules are included.
• TCP/IP stack and USB slave support.
• Based on GNU tools, open-source, popular.

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Larger systems

• VxWorks popular but expensive real-time
  operating system for many devices.
• Good suite of development tools for debugging the
  running system.
• High reliability code (widely used by aerospace
  and military).

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Mobile systems

• Symbian OS designed for phones.
• API for contacts, messaging, browsing
• TCP/IP stack (?)
• WAP, HTTP, SSL, XML and XHTML support.
• Communication Irda, Bluetooth, USB, Serial
• POP, Imap mail support
• Java VMs personal Java and J2ME

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Real-time OSes

• Hard-hat Linux
• Qnx
• Lynx
• Nucl(e)ar
• Tiny-OS

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Mobile systems

• Qualcomms BREW, aka Verizons Get It Now
• Similar features to Symbian, but allows binary
  applets to be downloaded from the server.
• Designed for native code or native browsers, e.g.
  they have Flash and SVG (2d graphics).
• Not really an OS, but a set of APIs for device
  services.

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Network protocol stacks

• Network software is deeply layered e.g. TCP/IP

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Bluetooth protocol stack

• Lets take a quick look at the Bluetooth stack

Software (on microcontroller)
Bluetooth radio module
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Bluetooth protocol stack

• These (core) layers are always present
• HCI The Host Controller Interface layer provides
  a standard communications protocol between the
  stack and the Bluetooth module. HCI communication
  packets can be transmitted using UART, RS232 or
  USB interface.
• L2CAP The Logical Link Control and Adaptation
  Protocol layer allows multiple channels to share
  a single Bluetooth link. It also handles
  segmentation and assembly of long messages, group
  management and quality of service functionalities.

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Bluetooth protocol stack

• These core layers are always present
• RFCOMM The RFCOMM layer implements the
  functionalities of a virtual RS232 link. Most of
  the application Profiles uses RFCOMM as a means
  of transmitting and receiving data.
• SDP The Service Discovery Protocol layer provides
  functionalities to publish supported Bluetooth
  functionalities (SDP server), as well as for
  querying remote Bluetooth devices for supported
  functionalities (SDP client).

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Bluetooth protocol stack

• The higher layers implement one or more
  bluetooth profiles with specific APIs
• Bluetooth audio
• Bluetooth printing
• Data access profiles (incl. TCP/IP over
  Bluetooth)
• Information transfer profiles (calendar, contacts
  sync.)
• Human Interface device profiles (mouse,
  keyboard,)
• Overall, this is a lot of code! But many
  Bluetooth stack implementations exist, including
  open source stacks. E.g. BlueZ for Linux and
  Smart-Its for Atmel AVR chips.

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CAN stack

• CAN stack is relatively simple, possibly built
  into hardware.
• Another protocol stack called Devicenetis built
  on the CANphysical layer.

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Direct communication

• Note that standard protocols are designed to
  allow devices to communicate with other devices
  of unknown origin.
• If all your devices are configured by you, you
  can bypass the protocol and use low-level
  protocols directly (like RS485).