What are embedded systems? Give a definition.
- “Dortmund” Definition (Peter Marwedel):
Embedded systems are information
processing systems embedded into a larger product. - An alternative definition can be given in terms of common characteristics an
embedded system is required to have.
How can cyber-physical systems be defined?
- Cyber-Physical Systems are integrations of computation with physical processes
- Cyber-physical system (CPS) = Embedded System (ES) + physical environment
What are common characteristics of embedded systems and cyber-physical systems?
Systems fulfilling most characteristics listed below are called embedded systems:
• They have to be dependable. Dependability encompasses reliability, maintainability, availability, safety and security.
• Efficiency in terms of energy, code size, run-time, weight and cost.
• Interaction with physical environment through sensors and actuators
• Many embedded systems have real-time constraints (soft & hard). Failing to
complete computations within a given time-frame can result in a loss of service
quality or cause harm to the user.
• Embedded systems are usually reactive systems in the sense that they may need
to continually interact with their environment and react on input from that
environment.
• Many embedded systems consist of analog and digital parts making them hybrid
systems.
• Dedicated user-interface in contrast to a classical desktop-PC with a keyboard,
mouse and monitor.
• Dedicated for a specific task in contrast to a desktop-PC running a wide variety
of different or general purpose programs (office software, games).
List examples for embedded systems and cyphy system types.
Automotive Domain • ABS: Anti-lock braking systems • ESP: Electronic stability control • Airbags • Efficient automatic gearboxes • Theft prevention with smart keys • Blind-angle alert systems • Adaptive cruise control
Avionics • Flight control systems Page 2 • Anti-collision systems • Pilot information systems • Power supply system • Flap control system • Entertainment system • Cabin control system
Surrounding Systems which incorporate embedded systems for specific tasks:
• Forestry Machine (dedicated embedded system to control arms and tools)
• Aircraft (Flight control system)
List and describe four requirements or features for specification and modeling languages.
• Component-Based Design: It must be “easy” to derive the behavior of a system
from the behavior of its components. If two components are connected, the resulting
new behavior should be predictable.
• Hierarchy: Human beings are generally not capable of comprehending systems
containing many objects (states, components) having complex relations with each
other. The description of all real-life systems needs more objects than human beings
can understand. Hierarchy (in combination with abstraction) is a key mechanism
helping to solve this dilemma. Hierarchies can be introduced such that humans need
to handle only a small number of objects at any time.
• Timing: Many embedded systems are real-time systems. Therefore, explicit timing requirements are one of the characteristics of embedded systems. The need for
explicit modeling of time is even more obvious from the term “cyber-physical system”. Time is one of the key dimensions of physics. Hence, timing requirements
must be captured in the specification of embedded/cyber-physical systems.
• Event handling: Due to the reactive nature of embedded systems, mechanisms
for describing events must exist. Such events may be external events (caused by the environment) or internal events
Describe which kind of states exist. Which one allows concurrent execution?
- Each state which is not composed of other states is called a basic state
- States containing other states are called super-states.
- Super-states B are called OR-super-states, if exactly one of the sub- states of
B is active whenever B is active. - Super-states B are called AND-super-states if the system containing B will
be in all of the immediate sub-states of B whenever it is in B –> allows concurrent execution
What are the StateMate semantics, are they determinate?
- Effect of external changes on events and conditions is evaluated.
- The set of transitions to be made in the current step and right hand sides of
assignments are computed. Results are assigned to temporary variables. - Transitions become effective, variables obtain new values from temporary variables
Due to the separation of computing new values (phase 2) and assigning them (phase
3), a StateCharts model with StateMate semantics will always produce the same
results for the same inputs when all transition conflicts are resolved and no undefined
behavior exists —> determinate
Name the three types of Petri nets discussed in the lecture! Give a short explanation
for each type.
- C/E (Condition/Event) Nets: simple nets with no isolated elements meeting some additional restrictions
- Place/Transition Nets: Places have capacity K (default: infinity), weight (how much consumed/produced), has indistinguishable token
- Predicate/Transition Nets: tokens are individuals (distinguishable), transitions if function at incoming edge true
used for more complex systems
One of the petri net types allows several non-distinguishable tokens per place. Which components are used in a mathematical model of such nets?
- place/transition nets
- marking M(p), weight W(p,t), places p and transitions t
Which type of communication is used in the following languages: StateCharts, SDL,
VHDL, CSP, Petri nets?
Shared: StateCharts, VHDL
Synchronous: CSP (Communicating Sequential Processes), Petri-Nets
Asynchronous: SDL, Petri-Nets
What is the so called powerwall? Describe and name solutions.
- one of the problems when the computational power/performance shall be increased
- static power consumption is mostly dependent on the leakage current of transistors
- dynamic power consumption:
Pdyn ~ Vdd^2
f ~ Vdd
Pstat ~ 1/A (A:area of transistor gate) - power density increases with shrunken structure size of CMOS-transistors -> increased leakage and heat produce
Countermeasures:
- voltage scaling
- dynamic power management
- introduction of parallelism
- reduction of code size
- reduction of run-time
What is the so called memory wall? Describe and name solutions.
The performance of processors had increased much faster than that of memories. This drifting has more or
less stopped nowadays but the gap in performance between processors and memories
still exists.
Countermeasures:
- use fast and small memories (cache/scratchpad)
- location close to processor (minimize access time)
One important properties of communication, especially in embedded systems with hard
real-time constraints is robustness. Name and describe
common means that are used to achieve this property!
Robustness in terms of communication means that external influences (e.g. noise)
do not have any significant impact on the communication. One common way to
achieve this is to use differential signals instead of single-ended ones.
- most noise added to both signals, using the difference cancles most noise
- another advantage of differential signal: change of Vdd (5V -> 3.3V) soes not have a big influence
- larger range -> higher transfer rate
One important properties of communication, especially in embedded systems with hard
real-time constraints is real-time behavior. Name and discribe how this is achieved.
Real-time behavior means, that each device connected to a bus is allowed to communicate in a guaranteed time and does not starve. The two most used methods to
achieve this are Carrier Sense Multiple Access (CSMA) and Time Division Multiple
Access (TDMA - signal multiplexing).
What is TDMA?
- time multiplex for a single bus
- every bus member gets assigned a fixed time slot in which it can send
- ‘round-robin’
What is CSMA, what form exist?
Carrier Sense Multiple Access. Two forms exist, Collision Avoidance (CA) and Collision Detection (CD).
First step is for both types the same: Check if bus is idle.
CA: if bus full wait random amount of time, otherwise send full message
CD: if bus is free send message bit by bit, cheking after each transmission if collision occured. If it occured, the task wait random time and counter increases (if counter reaches maximum transmission is aborted)
