IT606 Embedded Systems Software

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IT-606Embedded Systems(Software)

• S. Ramesh
• Kavi Arya
• Krithi Ramamritham
• KReSIT/ IIT Bombay

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Esterel Advanced Features

• S. Ramesh

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Multiform notion of time

• Belt Controller Specification
• " Five seconds after the key is turned on, if the
  
• belt has not been fastened, an alarm will beep
  for five seconds or until the key is turned off"

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Declaration

• module belt_control
• input reset, key_on, key_off, belt_on, end_5,
• end_10
• output alarm(boolean), start_timer

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Module body

• loop
• abort
• emit alarm(false)
• every key_on do
• abort
• emit start_timer await end_5
• emit alarm(true) await end_10
• when key_off or belt_on
• emit alarm(false)
• end
• when reset
• end

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• module timer
• constant count_5 integer
• input msec, start_timer
• output end_5, end_10
• every start_timer do
• await count_5 msec
• emit end_5
• await count_5 msec
• emit end_10
• end .

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Instantaneous dialogue

• module inst_dialogue
• input I
• output S
• signal Q, Y in
• await tick
• present I then emit Q end
• present Y then emit S end
• ?
• every Q do emit Y end
• end module

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• Input I triggers local Q
• Q triggers Y
• Y triggers S
• all happen instantaneously
• in the same reaction

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Another example

• P emit are_u_ready
• present I_am_ready then
• emit success
• stat1
• else
• emit fail
• stat2

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• Q loop
• do
• loop
• emit I_am_ready
• each are_u_ready
• stat3
• upto change
• do
• stat4
• upto change
• end

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Use of Broadcasting

• Simplifies communication among concurrent modules
  
• Sender does not wait for the receiver
• Receiver does not care who the sender is
• Essentially like a shared variable but without
  associated problems
• Works because of synchrony hypothesis!
• Other problems creep in, of course!

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Causality Problems

• Non-reactive
• signal S in
• present S then
• nothing
• else
• emit S
• end
• end
• What is the behavior of this program?
• S is present iff it is not present

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Nondeterminism

• signal S in
• present S then
• emit S
• else
• nothing
• end
• end
• S is emitted and is present
• or it is not emitted and it is not present
• both status are possible!

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Logical Correctness

• A program is logically correct if it is
  deterministic and reactive.
• Accepted programs should be logically correct
• The above programs are not logically correct and
  rejected by the compiler
• Some more additional restrictions
• They have to be acyclic

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Dependency Cycles

• present O1 then emit O2 end
• present O2 else emit O1 end
• O2 is emitted if O1 is present
• O1 is emitted if O2 is not present
• cyclic dependency
• rejected by the compiler!

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More Acyclic programs

• Is the following program logically correct?
• input I
• signal O1,O2 in
• present I then
• present O1 then emit O2 end
• else
• present O2 then emit O1 end
• end
• end

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Cycles

• What about this?
• present O1 then emit O2
• pause
• present O2 then emit O1
• They are accepted by the compiler!
• Compile-time cycles does not matter
• No run-time cycles allowed.
• Programs should be Constructively causal

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Constructive Causality

• present O1 then emit O1 end
• present O1 and not O2 then emit O2 end
• logically correct program but rejected
• It is not causal

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O1 O1 O2 O1 and not O2
O1
O1 0 O1 1 ?
O2
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Causality

• present S then emit S else emit S
• Is this equivalent to emit S?
• The former is rejected!
• The causality notion non trivial,
• More restrictive than logical correctness
• Status of local signals should have unique
• values at every reaction step
• Status determinable by a constructive
• deduction process