Fundamentals of Control Systems and Optimization in Embedded Systems

1. ELN5622Embedded SystemsClass 9Spring, 2003 Aaron Itskovichitskova@algonquincollege.com

2. Outline • Control systems/robotics • Sensors • Actuators • Feedback (Open loop vs. closed loop) • System performance • Profiling • Optimization • Speed vs. size • Bottlenecks • Stack depth • Iterative enhancements

3. Control systems introduction • What is control system?

4. Control systemsIntroduction • Control design specifications provided by the user usually come asa vague set of wants,e.g. • The door has to open before a person can walk into the glasspanel. • The train has to slow down enough to stop before theend of the track. • The same output power should be supplied to the transmittereven during changes in supply voltage. • Vibration of the read head of the CD reader due to a car drivingon a normal road should not cause the CD to lose track.

5. Open loop vs. Closed loop Open loop Closed loop

6. Control system example • House temperature control • Sensors • Temperature • Humidity • Actuators • Furnace on/off • Fan • Air conditioner on/off

7. Sensors • Temperature • Contact: Thermocouple, resistance based, thermistors • Non contact: Infrared specter, Optical specter • Distance (Infrared, ultrasound, radio) • Motion/position indication • Image recognition/machinevision • Force (etc pressure)

8. Actuators • DC motor control (PWM) • Step motor • Servo motor • Switches • MEMS

9. Optimization • Things to consider when programming : • Register, cache and main memory utilization • System timing constraints • Variable optimization • Number of branches and function calls • Stack depth • Resulting application size

10. Assembly may be required • In order to improve the run-time efficiency of some algorithms, it may be necessary to code them in assembly language • This practice may also be used in an effort to condense the space required to store the program data in the system memory

11. count1( unsigned char c) { unsigned char mask = 1; unsigned char count = 0; while (c) { if (c && mask) count++; mask = mask << 1; } return count; }

12. pshx ; allocate 2 byte auto variable des ; allocate 1 byte auto variable pshy ; Save stack frame tsy ; Set current stack frame ldx *ZD5 pshx ; pushed register *ZD5 ;;;END PROLOGUE ldab *ZD0+1 stab 2,y ; movqi: *ZD0 -> 2,y ldab #1 stab 3,y ; movqi: #1 -> 3,y ldab #0 stab 4,y ; movqi: #0 -> 4,y L2: ldab #0 stab *ZD5+1 ; movqi: #0 -> *ZD5 tst 2,y ; tstqi: MEM:2,y bne .+5 jmp L5 ; (beq) long branch ldab #1 stab *ZD5+1 ; movqi: #1 -> *ZD5 ; END L5: ldab *ZD5+1 ; tstqi: R:*ZD5 beq .+5 jmp L4 ; (bne) long branch jmp L3 L4: ldab #0 stab *ZD5+1 ; movqi: #0 -> *ZD5 tst 2,y ; tstqi: MEM:2,y bne .+5 jmp L7 ; (beq) long branch tst 3,y ; tstqi: MEM:3,y bne .+5 jmp L7 ; (beq) long branch ldab #1 stab *ZD5+1 ; movqi: #1 -> *ZD5 L7: ldab *ZD5+1 ; tstqi: R:*ZD5 bne .+5 jmp L6 ; (beq) long branch ldab 4,y addb #1 stab 4,y ; addqi3: 4,y by #1 -> 4,y

13. L6: ldab 3,y aslb stab 3,y ; ashlqi3: 3,y by #1 -> 3,y jmp L2 L3: ldab 3,y clra std *ZD5 ; zero_extendqihi2: 3,y -> *ZD5 ldd *ZD5 std *ZD0 ; movhi: *ZD5 -> *ZD0 jmp L1 jmp L1 jmp L8 jmp L1 L8: L1: ;;;EPILOGUE pulx ; Pulling register *ZD5 stx *ZD5 puly ; Restore stack frame pulx ; deallocate 2 byte auto variable ins ; deallocate 1 byte auto variable rts ; return from function ;;;----------------------------------

14. What to optimize • In order to enhance the performance of the system, we must first determine where it is deficient! • We get this information by performing tests on the system • The tests must be representative applications or kernels of the representative applications

15. Profiling • One method of finding the performance of the system is to profile its performance • In software, each instruction, instruction block, loop or function may be analyzed gather critical information • This information may include frequency of execution, execution time in a loop, or latencies associated with function calls • This information provides the designer with detailed information to perform application tuning

16. Bottleneck Analysis • One consequence of the Forced Flow Law is that the device utilization Ui is directly proportional to the service demand being placed on the device Di. • In more practical terms, if the memory system has the slowest service time but it has a high demand (large number of accesses) placed on it, it becomes the performance limiting factor for the entire system.

17. Iterative performance enhancement • In general, as you proceed to fix each bottleneck, you will undoubtedly discover new ones • Performance enhancements for embedded systems may become an iterative process • This is especially true if the system has to perform tasks within strict time bounds

18. Bottleneck elimination • As a general rule, the largest system bottleneck should be eliminated first • Once this is done, each newly discovered bottleneck must be recursively eliminated • Remember, sometimes a hardware bottleneck is caused by poor programming practices • The embedded system designer must be mindful of all system aspects (hardware and software)

19. Software optimisation • Speed vs. size • Lookup vs. search • Table based approach • Loop transformation • branching • Interrupt vs. Polling

20. Hardware optimizations • DMA (burst, cycle stealing, ..) • Buffers (FIFO) • Co-processors • Floating point Arithmetic • Communication