1. Realtime Embedded System Design
B. Ramamurthy
4/7/2019

2. Example real-time and embedded systems
Domain
Application
Avionics
Navigation; displays
Multimedia
Games; simulators
Medicine
Robot surgery; remote surgery; medical imaging
Industrial systems
Robot assembly lines; automated inspection
Civilian
Elevator control
Automotive system; Global positioning system (GPS)
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3. Lets discuss some realtime system (RTS) characteristics
Realtime response
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4. Realtime Characteristics
RTS have to respond to events in a certain pre-detemined amount of time.
The time constraints have to be considered during planning, design, implementation and testing phases.
Internal failures due to software and hardware fault have be handled satisfactorily.
You cannot simply pop-up a dialog error box that says “send report” or “don’t send report”.
Also external failures due to outside sources need to be handled.
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5. Realtime Characteristics (contd.)
Typical interaction in an RTS is asynchronous. Thus an RTS should have features to handle asynchronous events such as interrupt handlers and dispatcher and associated resources.
Potential for race condition: when state of resources are timing dependent race condition may occur.
Periodic tasks are common.
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6. Embedded System
Is a special purpose system designed to perform a few dedicated functions.
Small foot prints (in memory)
Highly optimized code
Cell phones, mp3 players are examples.
The components in an mp3 player are highly optimized for storage operations. (For example, no need to have a floating point operation on an mp3 player!)
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7. Real-time system concepts
A system is a mapping of a set of input into a set of outputs.
A digital camera is an example of a realtime system: set of input including sensors and imaging devices producing control signals and display information.
Realtime system can be viewed as a sequence of job to be scheduled.
Time between presentation of a set of inputs to a system and the realization of the required behavior, including availability of all associated outputs, is called the response time of the system.
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8. Real-time system concepts (contd.)
Real-time system is the one in which logical correctness is based on both the correctness of the output as well as their timeliness.
A soft real-time system is one in which performance is degraded by failure to meet response-time constraints.
A hard real-time system is one in which failure to meet a single deadline may lead to complete and catastrophic failure.
More examples:
Automatic teller: soft
Robot vacuum cleaner: firm
Missile delivery system: hard
Given a system you should be able to classify it.
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9. Embedded Systems
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10. Requirements-Engineering Process
Deals with determining the goals, functions, and constraints of systems, and with representation of these aspects in forms amenable to modeling and analysis.
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11. Types of requirements
Standard scheme for realtime systems is defined by IEEE standard IEEE830.
It defines the following kind of requirements:
Functional
Non-functional
External interfaces
Performance
Logical database
Design constraints (ex: standards compliance)
Software system attributes
Reliability, availability, security, maintainability, portability
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12. Simple kernels
Polled loop: Say a kernel needs to process packets that are transferred into the DMA and a flag is set after transfer:
for(;;) {
if (packet_here) {
process_data();
packet_here=0; }
Excellent for handling high-speed data channels, a processor is dedicated to handling the data channel.
Disadvantage: cannot handle bursts
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13. Simple kernels: cyclic executives
Illusion of simultaneity by taking advantage of relatively short processes in a continuous loop:
for(;;) {
process_1();
process_2();
process_3(); …
process_n(); }
Different rate structures can be achieved by repeating tasks in the list:
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14. Cyclic Executives: Example: Interactive games
Space invaders:
for(;;) {
check_for_keypressed();
move_aliens();
check_collision();
update_screen(); }
check_keypressed() checks for three button pressings: move tank left or right and fire missiles.
If the schedule is carefully constructed we could achieve a very efficient game program with a simple kernel as shown above.
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15. Interrupt driven systems
Main program is a simple loop.
Various tasks in the system are schedules via software or hardware interrupts;
Dispatching performed by interrupt handling routines.
Hardware and software interrupts.
Hardware: asynchronous
Software: typically synchronous
Executing process is suspended, state and context saved and control is transferred to ISR (interrupt service routine)
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16. Interrupt driven systems: code example
void main() { init(); while(TRUE); } void int1(void){ save (context); task1(); retore (context);} restore (context);}
Foreground/background systems is a variation of this where main does some useful task in the background;
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17. Design methods: Finite state machines
Finite state automaton (FSA), finite state machine (FSM) or state transition diagram (STD) is a formal method used in the specification and design of wide range of embedded and realtime systems.
The system in this case would be represented by a finite number of states.
Lets design the avionics for a drone aircraft.
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18. Finite State Machine (FSM)
M = five tuple  { S, i, T, Σ, δ }
S = set of states
i = initial state
T = terminal state (s)
Σ = events that bring about transitions
δ = transitions
Lets do this exercise for the avionics for fighter aircraft
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19. Drone aircraft avionics (simplified)
else
MA: Mission Assigned
TD: Target Detected
LO: Locked On
EE: enemy Evaded
ED: Enemy Destroyed
MC: Mission Complete
else
TAK MA
NAV TD
else
NAE MC
TAK: Take off
NAV: Navigate
NAE: Navigate & Evade
NAA: Navigate & Attack
LAN: Land
else LO
NAA EE
LAN ED
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20. State Transition tableMA LO TD MC EE ED
TAK
NAV
NAE
LAN
NAA
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21. Lets design a simple embedded/ realtime system
Use the table to code a function with case/switch statement
Or write a table-driven code
Which is better and why?
Lets implement this. I will show a demo later.
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22. Finite state automata and Co-routine based kernels
void process_a(void){ for(;;) { switch (state_a) { case 1: phase_a1(); | case 2: phase_a2(); | …. case n: phase_an();}}} void process_b(void){ switch (state_b) { case 1: phase_b1(); | case 2: phase_b2(); | case n: phase_bn();}}}
state_a and state_b are state counters;
Communication between coroutines thru’ global variables;
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23. Summary
We learned that realtime system requirements are quite different from that of regular systems.
Discussed many approaches to designing realtime systems.
Studied FSM based design of realtime systems
Demo
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