Embedded System Design

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Embedded System Design
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Embedded System Design
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Specification
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Requirements for specification techniques (1)

• HierarchyHumans not capable to understand
  systems containing more than 5 objects.Most
  actual systems require more objects
• Behavioral hierarchyExamples states, processes,
  procedures.
• Structural hierarchyExamples processors, racks,
  printed circuit boards
• Timing behavior
• State-oriented behavior
• Required for reactive systems
• classical automata insufficient.

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Requirements for specification techniques (2)

• Event-handling (external or internal events)
• No obstacles for efficient implementation
• Support for the design of dependable
  systemsUnambiguous semantics, ...
• Exception-oriented behavior
• Not acceptable to describe exceptions for every
  state.

We will see, how all the arrows labeled k can be
replaced by a single one.
.
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Requirements for specification techniques (3)

• Concurrency
• Real-life systems are concurrent
• Synchronization and communication
• Components have to communicate!
• Presence of programming elements
• For example, arithmetic operations, loops, and
  function calls should be available
• Executability
• Support for the design of large systems (? OO)
• Domain-specific support

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Requirements for specification techniques (4)

• Readability
• Portability and flexibility
• Termination
• It should be clear, at which time all
  computations are completed
• Support for non-standard I/O devices
• Direct access to switches, displays, ...
• Non-functional properties
• fault-tolerance, disposability, EMC-properties,
  weight, size, user friendliness, extendibility,
  expected life time, power consumption...
• Adequate model of computation

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Modeling and Design

• Modeling
• Act of representing a system or subsystem
  formally
• Mathematical model
• Viewed as a set of assertions about properties of
  the system such as its functionality or physical
  dimensions
• Constructive model
• Defines a computational procedure that mimics a
  set of properties of the system
• Executable models
• Design
• Act of defining a system or subsystem
• Defining one or more models of the system
• Refining the models until the desired
  functionality is obtained within a set of
  constraints

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Models of computation - Definition -

• How can we (precisely) capture behavior?
• We may think of languages (C, C), but
  computation model is the key
• Models of computation define Lee, UCB, 1999
• What it means to be a component
• Subroutine? Process? Thread?
• The mechanisms by which components interact
• Message passing? Rendez-vous?
• Possibly also
• What components know about each other(global
  variables? Implicit behavior of other components)

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Models vs. languages

• Computation models describe system behavior
• Conceptual notion, e.g., recipe, sequential
  program
• Languages capture models
• Concrete form, e.g., English, C
• Variety of languages can capture one model
• E.g., sequential program model ? C,C, Java
• One language can capture variety of models
• E.g., C ? sequential program model,
  object-oriented model, state machine model
• Certain languages better at capturing certain
  computation models

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Models and languages

• Common computation models
• Sequential program model
• Statements, rules for composing statements,
  semantics for executing them
• Communicating process model
• Multiple sequential programs running concurrently
• State machine model
• For control dominated systems, monitors control
  inputs, sets control outputs
• Dataflow model
• For data dominated systems, transforms input data
  streams into output streams
• Object-oriented model
• For breaking complex software into simpler,
  well-defined pieces

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Models of computation- Examples (1) -

• Communicating finite state machines (CFSMs)

• Discrete event model (VHDL, Verilog)

queue
a b c
time action
5 10 13 15 19
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Models of computation- Examples (2) -

• Differential equations (Analog circuits, Physical
  Systems)

• Asynchronous message passing (Kahn Process
  networks, dataflow models)

• Synchronous message passing (CSP, ADA
  rendez-vous)

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StateCharts recap of classical automata

• Classical automata

Moore or Mealy automata finite state machines
(FSMs)
Next state Z computed by function ?Output
computed by function ?
e1
Z0
Z1

• Moore-automataY ? (Z) Z ? (X, Z)
• Mealy-automataY ? (X, Z) Z ? (X, Z)

0
1
e1
e1
Z2
Z3
2
3
e1
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StateCharts

• Classical automata not useful for complex systems
• complex graphs cannot be understood by humans
• Introduction of hierarchy ? StateCharts Harel,
  1987

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Introducing hierarchy
FSM will be in exactly one of the substates of S
if S is active(either in A or in B or ..)
superstate
substates
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Definitions

• Current states of FSMs are also called active
  states.
• States which are not composed of other states are
  called basic states.
• States containing other states are called
  super-states.
• For each basic state s, the super-states
  containing s are called ancestor states.
• Super-states S are called OR-super-states, if
  exactly one of the sub-states of S is active
  whenever S is active.

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Default state mechanism

• Filled circle indicates sub-state entered
  whenever super-state is entered.
• Allows internal structure to be hidden for
  outside world.

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History mechanism

• For input m, S enters the state it was in before
  S was left (can be A, B, C, D, or E).
• If S is entered for the very first time, the
  default mechanism applies.
• History and default mechanisms can be used
  hierarchically.

(behavior different from last slide)
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Combining history and default state mechanism
same meaning
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Concurrency

• Convenient ways of describing concurrency are
  required.
• AND-super-states FSM is in all (immediate)
  sub-states of a super-state Example

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Entering and leaving AND-super-states

• Line-monitoring and key-monitoring are entered
  and left, when service switch is operated.

incl.
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Types of states

• In StateCharts, states are either
• basic states, or
• AND-super-states, or
• OR-super-states.

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Timers

• Since time needs to be modeled in embedded
  systems, timers need to be modeled.
• In StateCharts, special edges can be used for
  timeouts.

If event a does not happen while the system is in
the left state for 20 ms, a timeout will take
place.
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Using timers in answering machine
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General form of edge labels

• Events
• Exist only until the next evaluation of the model
• Can be either internally or externally generated
• Conditions
• Refer to values of variables that keep their
  value until they are reassigned
• Reactions
• Can either be assignments for variables or
  creation of events
• Example
• service-off not in Lproc / service0

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The StateCharts simulation phases

• How are edge labels evaluated?
• Three phases
• 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,
• Transitions become effective, variables obtain
  new values.
• Separation into phases 2 and 3 guarantees
  deterministic and reproducible behavior.

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Example

• In phase 2, variables a and b are assigned to
  temporary variables.
• In phase 3, these are assigned to a and b. As a
  result, variables a and b are swapped.
• In a single phase environment, executing the left
  state first would assign the old value of b (0)
  to a and b. Executing the right state first would
  assign the old value of a (1) to a and b. The
  execution would be non-deterministic.

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Reflects model of clocked hardware
In an actual clocked (synchronous) hardware
system, both registers would be swapped as well.
Same separation into phases found in other
languages as well, especially those that are
intended to model hardware.
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Steps
Execution of a StateChart model consists of a
sequence of (status, step) pairs
Status values of all variables set of events
current time Step execution of the three
phases
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Broadcast mechanism

• Values of variables are visible to all parts of
  the StateChart model.
• New values become effective in phase 3 of the
  current step and are obtained by all parts of the
  model in the following step.

? StateCharts implicitly assumes a broadcast
mechanism for variables. ? StateCharts is
appropriate for local control systems (?), but
not for distributed applications for which
updating variables might take some time (?).
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Lifetime of events

• Events live until the step following the one in
  which they are generated (one shot-events).

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Evaluation of StateCharts (1)

• Pros
• Hierarchy allows arbitrary nesting of AND- and
  OR-superstates.
• Semantics defined in a follow-up paper to
  original paper.
• Large number of commercial simulation tools
  available(StateMate, StateFlow, BetterState,
  ...)
• Available back-ends translate StateCharts into
  C or VHDL, thus enabling software or hardware
  implementations.

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Evaluation of StateCharts (2)

• Cons
• Generated C programs frequently inefficient,
• Not useful for distributed applications,
• No program constructs,
• No description of non-functional behavior,
• No object-orientation,
• No description of structural hierarchy.

• Extensions
• Module charts for description of structural
  hierarchy.