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Real Time System With Linux

Linux is being used more and more in embedded
devices such as cell phones, networking gear, and
even MP3 players. As software engineers work to
integrate the Linux run-time into these devices,
they often find that there can be several
challenges to meeting real-time requirements. So
what is real-time and how can we meet real-time
requirements using Linux?

4 © 2009 Wind River Systems, Inc.
Outline
• A matter of time
• What is Real-Time?
• Having a real hard time with hard real time?
• Additional topics

5 © 2009 Wind River Systems, Inc.
What is time?

6 © 2009 Wind River Systems, Inc.
Computer Time
• Clocks
– TOD Clock
– System Clock
• Timers
– Hardware Based
– Software Based
• Time Codes
– SMPTE
– IRIG

7 © 2009 Wind River Systems, Inc.
Problems with Time
• Time is not universal
– There is no fixed universal time. Time is relative to the observer.
– Time Dilation is explained in terms of both general and special
relativity
• What we mean by time is not universal
– Time can be relative to the rotation of the earth (TOD)
– Time can mean a fixed repeated interval (Periodic)
– Predictable Response Time
• Bounded (50 ms +-5us)
• Fixed (50 ms +- 0)
– Real-Time

8 © 2009 Wind River Systems, Inc.
Problems with Computer Time
• Drift
– When one clock falls out of sync with another clock
• Synchronization
– Network
– GPS
– Clock to the earth’s non-regular period
• Jitter Atomic Cesium Fountain NIST-F1 (1999)

9 © 2009 Wind River Systems, Inc.
What is a Real Time System?
• Time is essential for correctness
• What does this mean?
– “… performance deadlines on its computations and actions”1
– Reactive or Event Driven
– Time Driven
• My definition
– A system whose behavior must be coordinated with the
behavior of a real object.
1 Doing Hard Time, Douglass, Addison Wesley 199, p.58

10 © 2009 Wind River Systems, Inc.
Example Real Time Systems
• Traffic...