Chapter 13 Embedded Systems

1. Chapter 13Embedded Systems • Embedded Systems • Characteristics of Embedded Operating Systems • eCos

2. Embedded System • A combination of hardware and software designed to perform a dedicated function • Often, embedded systems are part of a larger system or product, • e.g., antilock braking system in a car • Embedded systems are tightly coupled to their environment  imposes real-time constraints by the need to interact with the environment

3. Examples of Embedded Devices

4. Differences from typical computer • A variety of interfaces • enable the system to measure, manipulate, and interact with the external environment • human interface may be as simple as a flashing light or as complicated as real-time robotic vision • Use of a diagnostic port for diagnosing the system • Use of special purpose hardware to increase performance or safety • Fixed-function or application-specific software

5. Roadmap • Embedded Systems • Characteristics of Embedded Operating Systems • eCos

6. Characteristics of Embedded OS • Using a general-purpose OS for an embedded system may not be possible • constraint of memory space • constraint of power consumption • real-time requirements

7. Characteristics of Embedded OS • Real-time operation • correctness of computation depends, in part, on the time at which result is delivered • Reactive operation • need to consider worst-case conditions (due to external events) in execution • Configurability • only the functionality needed for a specific application and hardware suite is provided

8. Characteristics of Embedded OS • I/O device flexibility • handles devices by using special tasks instead of integrating their drives into the OS kernel • Streamlined protection mechanisms • limited protection because tested software can be assumed to be reliable • Direct use of interrupts • general-purpose OS typically do not permit any user process to use interrupts directly

9. Developing anEmbedded OS • Two general approaches • Take an existing OS and adapt it for embedded purposes • Design a purpose-built OS solely for embedded use

10. Adapting an Existing OS • Examples include Windows, Linux •  familiar interfaces facilitate portability •  slower and less predictable than special purpose embedded OS •  not optimized for real-time and embedded applications  require considerable modification to achieve adequate performance • optimizes for the average case rather than the worst case for scheduling • assigns resources on demand and ignores semantic information about an application

11. Adapting an Existing OS • Need to add • real-time capability • streamlining operation • other specialized and necessary functionality for the given device

12. Purpose-Built Embedded OS • Typical characteristics include: • Fast and lightweight process or thread switch • Real time scheduling policy • Small size • Responds to external interrupts quickly (<10 s) • Minimizes intervals during which interrupts are disabled

13. Purpose-Built Embedded OS • Typical characteristics include: • Provides fixed or variable sized partitions for memory management as well as the ability to lock code and data in memory • Provides special sequential files that can accumulate data at a fast rate

14. Timing Constraints • To deal with timing constraints, the kernel: • Provides bounded execution time for primitives • Maintains a real-time clock • Provides for special alarms and timeouts • Supports real-time queuing disciplines, e.g., EDF • Provides primitives to delay processing and to suspend/resume execution

15. Roadmap • Embedded Systems • Characteristics of Embedded Operating Systems • eCos

16. eCosEmbedded Configurable OS • Open source, royalty-free, real-time OS • Most widely used embedded OS • Targeted at high-performance small embedded systems

17. eCos Configuration Tool • The eCos configuration tool is used to configure an eCos package to run on a target embedded system At each level, the configuration user may select only those components needed for the target application Items on the list can be expanded to provide a finer-grained menu of options

18. eCos Layered Structure • eCos consists of a layered set of components to achieve portability to different architectures and platforms

19. Hardware Abstraction Layer • Presents consistent API to upper layers and maps upper-layer operations onto a specific platform Same call but different implementations

20. eCos Kernel Design • Four main objectives: • Low interrupt latency • The time it takes to respond to an interrupt and begin executing an ISR • Low task switching latency • The time it takes from when a thread becomes available to when actual execution begins

21. eCos Kernel Design • Four main objectives: • Small memory footprint • Memory resources for both program and data are kept to a minimum by allowing all components to configure memory as needed • Deterministic behavior • Throughout all aspect of execution, the kernels performance must be predictable and bounded to meet real-time application requirements

22. eCos Kernel • eCos kernel provides the core functionality needed for developing multi-threaded applications • create and control threads • scheduling, e.g., multilevel queue • synchronization, e.g., semaphores • Some not included to make for a small kernel • memory allocation (in separate package) • device drivers (in separate packages)

23. eCos I/O System • Framework for supporting device drivers • A variety of drivers are available through the configuration package • device drivers provide the necessary functions for I/O, buffering and device control • Principle objective is efficiency with no unnecessary software layering or excessive functionality