Embedded Systems

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Embedded Systems

• John Regehr
• CS 5460, Dec 04

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Embedded Systems

• Account for gt99 of new microprocessors
• Consumer electronics
• Vehicle control systems
• Medical equipment
• Sensor networks

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Definitions of Embedded System

1. A special-purpose computer that interacts with
   the real world through sensing and/or actuation
2. Almost any computer that isnt a PC

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More definitions

• Microprocessor A regular CPU
• Microcontroller A small system on a chip that
  contains extra logic for interfacing with the
  real world
• Analog to digital and digital to analog
  converters
• Pulse width modulation
• Networks serial, I2C, CAN, USB, etc

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Embedded Characteristics

• Close interaction with the physical world
• Often must operate in real time
• Constrained resources
• Memory
• SRAM, DRAM, flash, EEPROM,
• Power
• CPU cycles

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More Characteristics

• Often there is no
• Virtual memory
• Memory protection
• Hardware supported user-kernel boundary
• Secondary storage
• Hard to upgrade once deployed
• Cost sensitive
• Per-unit cost often dominates overall cost of a
  product

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Important Difference

• Unlike PC software, embedded software is
  developed in the context of a particular piece of
  hardware
• This is good App can be tailored very
  specifically to platform
• This is bad All this tailoring is hard work

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CPU Options

• Create custom hardware
• May not need any CPU at all!
• 4-bit microcontroller
• Few bytes of RAM
• No OS
• Software all in assembly
• These are getting less popular

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More CPU Options

• 8-bit microcontroller
• A few bytes to a few hundred KB of RAM
• At the small end software is in asm, at the high
  end C, C, Java
• Might run a home-grown OS, might run a commercial
  RTOS
• Typically costs a few

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More CPU Options

• 16- and 32-bit microcontrollers
• Few KB to many MB of RAM
• Usually runs an RTOS vxWorks, WinCE, QNX,
  ucLinux,
• May or may not have caches
• Wide range of costs
• 32- or 64-bit microprocessor
• Basically a PC in a small package
• Runs Win XP, Linux, or whatever
• Relatively expensive in power and

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Axes of Variation

• Power
• Must run for years on a tiny battery (hearing
  aid, pacemaker)
• Unlimited power (ventilation control)
• Real-time
• Great harm is done if deadlines are missed
  (avionics)
• Few time constraints (microwave)

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More Axes of Variation

• Importance
• Device is safety critical (nuclear plant)
• Failure is largely irrelevant (toy, electric
  toothbrush)
• Upgradability
• Impossible to update (spacecraft, pacemaker)
• Easily updated (firmware in a PC network card)

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More Variation

• Cost sensitivity
• A few in extra costs will kill profitability
  (many products)
• Cost is largely irrelevant (military
  applications)
• The point Embedded systems are highly diverse,
  its hard to make generalizations in this domain

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Programming Languages

• Assembler
• No space overhead
• Good programmers write fast code
• Non-portable
• Hard to debug
• C
• Little space and time overhead
• Somewhat portable
• Good compilers exist

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More Languages

• C
• Often used as a better C
• Low space and time overhead if used carefully
• Java
• More portable
• Full Java requires lots of RAM
• J2ME popular on cell-phone types of devices
• Bad for real-time!

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RTOS

• Low end Not much more than a threads library
• High end Stripped-down version of Linux or WinXP
• Important qualities
• Predictability
• Reliability
• Efficiency
• Do Linux and XP have these?

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Whats Hard?

• Concurrency threads, bottom-halves, interrupts
• Debugging
• Often printf and gdb dont exist
• Creating correct software cant patch a
  pacemaker
• Getting a product out the door quickly
• Meeting resource constraints (power, RAM, etc.)

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Some Tradeoffs

• Advanced tools are expensive (10,000 per
  developer) but can help a lot
• Buying an OS helps but usually results in less
  efficient systems
• Complex software architecture provides rich
  functionality but may be slow and hard to debug

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Creating Embedded SW

• Pick
• Cheapest CPU that seems like itll work
• Appropriate language
• Appropriate OS
• Appropriate software architecture
• Appropriate simulation tools
• Code test repeat

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Cyclic Executive

• main()
• init()
• while (1)
• a()
• b()
• c()
• d()

Advantages? Disadvantages?
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Cyclic Exec. Variations
main() init() while (1) a()
b() a() c() a()

• main()
• init()
• while (1)
• wait_on_clock()
• a()
• b()
• c()

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Interrupts Events

• main()
• while (1)
• event_t e
• get_event()
• if (e)
• (e)()
• else
• sleep_cpu()

interrupt2() time_critical_stuff()
enqueue_event (non_time_critical)
Advantages? Disadvantages?
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Multithreading

• Threads are usually sleeping on events
• Highest priority thread runs except when
• Its blocked
• An interrupt is running
• It wakes up and another thread is executing in
  the kernel

Advantages? Disadvantages?
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Port Based Objects

• PBOs periodically invoked by OS
• No blocking synchronization
• Preemption may or may not be supported

• my_pbo()
• get_inputs ()
• // No side effects!
• compute ()
• store_results ()

Advantages? Disadvantages?
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Summary

• Very different from programming desktop apps
• Software architecture is worth thinking about in
  advance
• Embedded systems are fun