IT-606 Embedded Systems (Software)

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

Esterel: Advanced Features S. Ramesh

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"

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

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

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 .

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

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

Another example P:[ emit are_u_ready; present I_am_ready then emit success; stat1 else emit fail; stat2 ]

Q:loop do [ loop emit I_am_ready; each are_u_ready || stat3; ]; upto change; do stat4 uptochange end

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!

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

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!

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

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!

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

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

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

O1 = O1 O2 = O1 and not O2 O1 O1 = 0 O1 = 1 ? O2

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

IT-606 Embedded Systems (Software)