1. 9/14/2015 1 Introduction to Realtime Systems Chapter 1 R. Williams’ text B. Ramamurthy

2. 9/14/2015 2 Overview Realtime processing requires both parallel activities and quick response (See figure in p.2 of a juggler) Design for complexity management. (See figure in p.3) See examples like parking meter Microprocessors and realtime applications Definition of realtime systems Programming structures Response latency Relative speeds Timing issues Debugging real-time systems Input/output handling Deadlines (hard, firm and soft) Software quality assurance

3. 9/14/2015 3 Design for complexity management Choices: Structured analysis/structured design (SA/SD) Concurrent design approach for real-time systems (CODARTS) Finite state machines (FSM) Object-oriented design (OOD)

4. 9/14/2015 4 Microprocessor Examples: vending machines, mobiles phones, alarm systems, washing machines, motor car engine controllers, heart monitors, microwave ovens all operate using embedded microcontrollers running dedicated software. Microprocessors are the enabling hardware for realtime systems.

5. 9/14/2015 5 Lets define realtime (RT) systems Timing: RT systems (RTS) are required to compute and deliver correct results within a specified period of time. Ex: traffic light controller Interrupt driven: event-driven preemption; RTS are often involved with handling events. Events manifest themselves in terms of interrupt signals arising from the arrival data at an input port or ticking of a hardware clock, or an error status alarm.

6. 9/14/2015 6 RTS Definition (contd.) Low-level programming: RTS often deal with devices; C language is still a favorite for writing device drivers for new hardware. Specialized hardware: Most RTS work within, or at least close beside, specialized electronic and mechanical devices. Often closed loop systems. (See fig on p.6) Volatile data IO: Variables that change their value from moment to moment. RTS software must be structured to check for changes at the correct rate, so as not to miss a data update.

7. 9/14/2015 7 RTS Definition (contd.) Multi-tasking: RTS are often multitasking. Several processes cooperate to carry out the overall job. Divide RTS problem into tasks as a design strategy. Run-time scheduling: Separation of activities into tasks leads to question of task sequencing or scheduling. Moreover the external events and required response to these lead to run-time scheduling or dynamic scheduling. Unpredictability in inputs/stimulus: Being event-driven, RTS are at the mercy of unpredictable changes in their environment. Predictability response requirement! Life-critical code: failure to run correctly may result in death or at least injury to the user and/or others. Life-critical systems requires extra testing, documentation and acceptance trials.

8. 9/14/2015 8 Programming Structures Also known as control structures Sequence (SEQ) Iteration (IT) Branches guarded by selection statements (SEL) Parallel or concurrent instructions (PAR) Critical group of exclusive instructions (CRIT)

9. 9/14/2015 9 Relative Speeds Polling is a common method used in RTS for sensing events/inputs. When designing RTS Avoid polling input too slowly, you may miss events Avoid sampling to frequently (called aliasing), since it will make the process inefficient. Nyquist limit: The maximum frequency threshold is half the sampling rate referred to as Nyquist limit.

10. 9/14/2015 10 V2F: Voltage to Frequency A commonly used environmental monitoring arrangement involves a transducer being interfaced to a voltage-frequency converter. V2F unit converts voltage to frequency: larger voltage, higher frequency; lower voltage, lower frequency. Computer has to dedicate a single bit input port to accept the information in serial mode. Problem: converting this bit information into integer. Timing of the two branches in the following sequences are not balanced; run these two alternating.

11. 9/14/2015 11 Software Timing loop for 100 msec { if (input_bit) hcount++; else lcount++; } loop for 100 msec { if (!input_bit) lcount++; else hcount++; }

12. 9/14/2015 12 High Speed Timing Consider a laser range finder used in civil surveying, and battle field targeting. Distance is calculated by the timing the duration of flight. The Speed of light: s= 3 X 10 8 m/sec Target is 20km away, the pulse of light will travel 40km (to and from): d = 4 X 10 4 m Time taken: d/s = 4 X 10 4 / 3 X 10 8 = 130μs If the item surveys is only at 50m distance, the flight time will be reduced to 325ns. Regular 500MHz computer will not do for this. Solution: Clear a high speed counter when the pulse leaves and capture the counter value on return of the pulse.

13. 9/14/2015 13 Output timing Timing problems when controlling output devices such as motors. Cyclic, periodic output data. Several types of motors: stepper, DC servo, universal AC, induction AC, synchronous AC etc. For example sometimes we need to dispatch every 20ms with an accuracy of 50μs. Solution: need special hardware support: controller boards.

14. 9/14/2015 14 Machine IO Programmed IO Interrupt driven Direct memory access

15. 9/14/2015 15 Types of RTS Hard RTS: tight limits on response time, so that a delayed result is a wrong result. Ex: jet fuel controller and camera shutter unit Soft RTS: need to meet only time-average performance target. As long as most results are available before deadline the system will run successfully. Ex: audio and video transmission, single frame skip is fie, but repeated loss is unacceptable Firm RTS: somewhere between the two. Ex: Space station solar panel unit

16. 9/14/2015 16 Software Quality Assurance QA is especially important to RTS since many of these are deployed in life critical environments / situations. Patriot missile failure

17. 9/14/2015 17 Summary We studied key issues which make development of realtime software more challenging than desktop or traditional data processing applications. Timing is very critical for RTS input, output, computing and response. See the timing units of measurement on page 26. Read the reference papers given at the end of the chapter.