Verification of Embedded Application – Approaches to Perform Worst Case Execution Timing 1 1 Execution Timing K S Sreetha 06-Dec-2012 

• Introduction • Timing Analysis • Different Approaches • Static Analysis Agenda 2 2 • Dynamic Analysis • Conclusion • Q&A 

Introduction • In Industries - avionics, automobile, nuclear plant, space research, medical etc.- safety plays a vital role. • Usage of real time embedded applications in various industries is increasing 3 3 industries is increasing rapidly. 

Real Time Systems • The data is processed with respect to time. • Timing constraints is one of the most important issues in real-time embedded system designs. 4 4 

Real Time Systems • The real time systems are classified into three types based on the deadline. • Hard Real Time Systems • Soft Real Time Systems • Firm Real Time System 5 5 • Firm Real Time System 

WCET Why is Worst Case execution Timing Analysis (WCET) performed? 6 6 

WCET cont… • Objective - Software implementation execution does not exceed the allocated frame time (the time allocated for a frame/task). • The WCET provides the worst case execution time; this is 7 7 • The WCET provides the worst case execution time; this is the time when the system behaves in the worst scenario. 

WCET cont… 8 8 • The WCET gives the safe time that no underestimation is considered and tight time that a small overestimation is considered. • Best Case Execution Time (BCET) gives the best execution time of the system with the normal inputs. 

Static Analysis • Without execution of the code on the hardware. • Advantages: • It guarantees that worst-case timing path can be covered from overall analysis. 9 9 covered from overall analysis. • Dis Advantages: • Time consuming. • There is no execution; it is not possible to consider the hardware factors. 

Dynamic Analysis • Execution of the software on the target hardware or on an evaluation board • All abnormal conditions are considered. 10 10 • Inputs to be simulated or some prerequisites need to be satisfied. • Interrupts to be simulated or generated. 

Dynamic Analysis cont.. • The steps involved in this approach are mentioned in the below section. • Test Design/Analysis • Test Execution 11 11 • Test Execution • Integration Figure 2 

Dynamic Analysis cont.. Test Design/Analysis • The program representation is captured through control flow diagram. • The control flow diagram gives the pictorial representation of the control flow of the code. 12 12 representation of the control flow of the code. 

Selection of Inputs • Knowledge of the system is very important • Consider Max and Normal Values of input. 13 13 

Dynamic Analysis cont.. Test Execution: • The time could be captured in the oscilloscope, or any of the measuring instruments. 14 14 

Dynamic Analysis cont.. Integration: • The integration is the third and last step in this approach • Integrates the functions individual WCET to get the WCET of the entire software. 15 15 of the entire software. • Bottom to top integration 

Dynamic Analysis cont.. Function4( ) { If ( ) { // Path I Statement1; If function4 has two paths like 16 16 Statement1; Statement2; } Else { //Path J Statement2; Statement3; Statement3; } } 

Dynamic Analysis cont.. Function4 path Time(µs ) Path I (If condition ) 640 Path J (else condition) 800 17 17 As per this data table the worst time consuming path is “ELSE condition “Path J. 

Dynamic Analysis cont.. Function5 path Time(µs) Path I (If condition ) 1000 PathJ(else condition) 500 18 18 As per this data table the worst time consuming path is “IF condition “Path I. 

Dynamic Analysis cont.. Consider time for the function call = 320µs Execution Time (ET) for PATH d = WCET of Function4+Function call of 4 +executable of Function1 19 19 Function4+Function call of 4 +executable of Function1 = 800 + 200 + 320 = 1320 

ET for PATH h = WCET of Function5+Function call of 5 +executable of Function1 = 1000 + 200 +320 20 20 = 1530 ET for PATH b= WCET of PATH h+ Function call of 1 +executable of Function main 

Worst Case Timing Calculation • Worst case Time = maximum time path + ET of the Interrupts • Timing Margin (%) = (1- (Worst case Time /Total time allocated))*100 21 21 allocated))*100 • It is recommended that WCET usage should not exceed 50% of the allocated time margin 

Dynamic Analysis cont.. • Advantages • Deals with real run-time usage • Guaranteed that the worst-case time path has been covered 22 22 • Disadvantages • Manual errors • The approach is time consuming. 

Conclusion • Best feasible method to for the embedded software application. • Calculation allows for the expression of the most complex flows. • This approach can be used in safety critical software 23 23 • This approach can be used in safety critical software applications like aerospace, medical, and defense, etc 

Q&A 24 24 Q&A 

Thank you 25 25 

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