© S. Ramesh / Kavi Arya / Krithi Ramamritham IT-606 Embedded Systems (Software) S. Ramesh Kavi Arya Krithi Ramamritham KReSIT/ IIT Bombay

© S. Ramesh / Kavi Arya / Krithi Ramamritham Esterel: Advanced Features S. Ramesh

© S. Ramesh / Kavi Arya / Krithi Ramamritham 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"

© S. Ramesh / Kavi Arya / Krithi Ramamritham Declaration module belt_control: input reset, key_on, key_off, belt_on, end_5, end_10; output alarm(boolean), start_timer;

© S. Ramesh / Kavi Arya / Krithi Ramamritham 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

© S. Ramesh / Kavi Arya / Krithi Ramamritham 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.

© S. Ramesh / Kavi Arya / Krithi Ramamritham 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

© S. Ramesh / Kavi Arya / Krithi Ramamritham Input I triggers local Q Q triggers Y Y triggers S all happen instantaneously in the same reaction

© S. Ramesh / Kavi Arya / Krithi Ramamritham Another example P: [ emit are_u_ready; present I_am_ready then emit success; stat1 else emit fail; stat2 ]

© S. Ramesh / Kavi Arya / Krithi Ramamritham Q: loop do [ loop emit I_am_ready; each are_u_ready || stat3; ]; upto change; do stat4 upto change end

© S. Ramesh / Kavi Arya / Krithi Ramamritham 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!

© S. Ramesh / Kavi Arya / Krithi Ramamritham Causality Problems Non-reactive: signal S in present S then nothing else emit S end What is the behavior of this program? S is present iff it is not present

© S. Ramesh / Kavi Arya / Krithi Ramamritham Nondeterminism signal S in present S then emit S else nothing end S is emitted and is present or it is not emitted and it is not present both status are possible!

© S. Ramesh / Kavi Arya / Krithi Ramamritham 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

© S. Ramesh / Kavi Arya / Krithi Ramamritham 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!

© S. Ramesh / Kavi Arya / Krithi Ramamritham 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

© S. Ramesh / Kavi Arya / Krithi Ramamritham 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

© S. Ramesh / Kavi Arya / Krithi Ramamritham 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

© S. Ramesh / Kavi Arya / Krithi Ramamritham O1 O2 O1 = O1 O2 = O1 and not O2 O1 = 0 O1 = 1 ?

© S. Ramesh / Kavi Arya / Krithi Ramamritham 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