Introduction to Real Time Systems Video Lecture | Embedded Systems (Web) - Computer Science Engineering (CSE)

hello everyone in this video I will
explain real-time system
what are real-time system and what are
the different types of freedom system so
before discussing real-time system just
have a look on this term so if we divide
our real-time system term so we will get
2 term real-time and system so you might
be wondering what's the real-time and
what is unreal time so first of all we
will see what is real-time so we are
having a click to type of motion for
time we can represent time in two
different ways
so first way is quantitative and second
is qualitative so quantitative whenever
you are representing a time with the
help of numbers so when you revisit a
time in numeric way that is known as
quantitative motion let us say for
example when we explain like this system
has to perform within 2 millisecond so
here we are specifying numeric so this
is quantitative and second one is
qualitative so here time is represented
with the help of words like before after
like if a query is given to a system
that system will produce output after
that query is processed so here this
notion will be using words whereas
quantitative will be using numeric so in
quantitative notion how this numeric
time can be achieved so they will be a
physical clock and time is quantified
with the help of that clock let's say
for example nowadays in every car we are
getting airbags
so whenever a accident occurred so that
airbag has to be opened within a
microsecond so
specify that after after accident after
lapsing three microseconds this bag has
to be open so there has to be a physical
clock and qualitative we have seen
example like query Vanek user is giving
query to a system that system has to
produce its output once it saved that
query so these were actually specify the
sequence of operation there is no
particular numeric number used now let's
see what is real-time system so before
going to discuss real-time system just
discuss a system normally when we have a
system how we going to identify whether
that system is working properly or not
we will use that system and output given
by that system we will compare that
output with the actual output so if both
compared it means that system is working
perfectly but in case of real-time
system first thing you need to remember
time this system actually in this system
time is used to describe behavior of
system and which notion of time a time
is used quantitative motion okay so in
real-time system
quantitative notion of time is used to
describe behavior of a system how let's
take this example let us say we are
having a calculator and we want to see
whether this calculator is functioning
properly or not so what we will do we
will give some input to calculator this
calculator will give us some output and
we will compare that output with the
actual output so we know that 2 plus 2
is equal to 4 and calculator also giving
this output both are same same way we
will do some other operation as well so
if all output match we can conclude that
our calculator is working properly so
here a system's correctness depend on
functioning of system only
but in real-time system a system is
considered correct if that system is
performing correctly with some time
constraint so when this correctness is
criteria include functioning plus timing
constraint those system actually known
as real time system like airbag
deployment system that system has to be
active within one or two milliseconds so
there will be a time constraint so if
that system is working after that time
constraint that system will be
considered fail so if after 10
millisecond of accident airbag open in
that case this system will be considered
filled so here now it new criteria is
defined for the correctness of system
time plus functioning this is the
architecture of real-time system we are
having sensor this sensor will sense and
provide input to input conditioning unit
now this signal provided by sensors or
data provided by sensors are very low
their voltage level is very low so this
input conditioning unit it will amplify
those signals and provided two input
interface and through this interface
real-time computer is connected so here
all the processing is going on and this
one is connected with SC i2 which is a
human can interact so similar we are
having four output interface outer into
output interface will provide signal to
output conditioning unit this unit will
convert those signal according to
actuator and then these actuator
actually will perform actual work now
let's discuss these two features theft
safety and reliability so when a system
is safe and when a system is not
if so if a system is safe and if failure
does not lead to any catastrophic
situation this one has to be followed in
system and the reliability what is
reliability reliability is ability to
perform its intended task under static
condition or stated duration like if you
are buying a system and that system
supposed to be functioning for five
years so if that system is working
perfectly for five years it can be
considered a liability so reliability
how long that system is working
perfectly in case of failure as well so
in traditional system safety and
reliability are two independent issues
but in real-time system the both are
covered how so a failsafe state so
whenever a system is failed at that time
in order to avoid all damage there will
be a state and so whenever there is a
failure system will enter into that
state that state is known as phase if
state now you need to remember this that
for all real-time system we don't have
failsafe state for some real-time system
there will be this state but for others
there won't be any fail-safe state like
we are having a gamble of traffic light
system so whenever this traffic light
system fails system will enter into a
failsafe state how it will start
blinking orange light so when anyone
will see that orange light that person
would know a traffic control is not
working so proceed with caution so there
is a fail-safe estate but in case of
anti-missile system in this case if this
system fail that would result in a
catastrophic
that would made a huge damage so here
for anti-missile system there is no
failsafe state once a missile is long
and if there is any problem with this
anti-missile system with missile
interceptor or any kind of problem
occurred this will be missed and it
would result in a huge damage so there
is no failsafe state so these type of
system known as safety critical system
system for which there is no failsafe
state exist those system is known as
safety critical system now the question
is that if there is no failsafe state
how to make sure that that system would
not make any damage so the only solution
is that we need to increase reliability
of those system so how can we increase
this reliability when we are having a
system there are chances that any
component of that state system stopped
functioning or the program stored in
that system stopped functioning there
can be a bug so there are multiple ways
first error avoidance so it state that
every possibility of error should be
minimized during product development but
it cannot be achieved like whenever a
system is in development some errors are
left
second one is error detection and
removal so this can be achieved like
system can be tested again and again and
then on the basis of test result that
system can be updated and third one
fault tolerance so the thing is that we
cannot create a system 100% error
there will be error so if that system is
capable of handling its error on its own
so in that case reliability can be
achieved
so we are having two type of fault
tolerance for wire one for hardware
another for software so for hardware
fault tolerance first approach is that
built in self test so this component
will test every other component in a
system so on the detection of a failure
whenever it will find the one component
is not working perfectly it will switch
out that component and it will switch in
another good component so for that case
for every component there will be a
redundant copy so whenever one pump on
and stopped working
another component will be used and the
second approach triple modular
redundancy so it it it state that for
every component three redundant copies
of those will be used to run
concurrently now let us say in a
component sorry in a system that system
is performing some tasks and in that
system for every critical component they
are three copies so whenever that task
is completed at the end voting processes
started so it will be seen that out of
three if to copy giving one output and
one copy giving another output so it
will be in the favor of those two copy
now then there is a question like how to
achieve software fault tolerance can be
used same techniques which are used in
Hardware fault tolerance so the question
is saw answer is that you cannot use
same technique so
so for software fault tolerance there
are number of techniques we are going to
see only two so first one in inversion
programming so in inversion programming
for every module different different
team will work on that module and they
will provide different module for same
tasks and whenever a task has to be
completed both all those third module
will be used and at then voting of
result will decide correctness of the
system so this is an version program and
second one recovery block so are in
recovery block for every module there
will be a redundant block and that is
also known as try block and every try
block compute same result but
intentionally written using different
algorithm so intentionally during
development every team will use
different algorithm to calculate same
result so this approach is recovery
block now let's have a look on types of
real-time system so how these categories
are actually made so these category a
firstly the example of this particular
view time system airbag deployment so we
know that if this airbag deployment
system means its deadlines like if this
system does not work in the particular
time period it will be considered fade
so what are the consequences missing
deadlines system will fail and after
those time consideration if that system
is performing there is no user not
accept it now let's see the another
example satellite based tracking so in
this example satellite will take picture
of enemy territory and send it to down
station now
let's say
the ground station is busy in processing
received images and meanwhile in new
images are received from the satellite
and that system actually missed to
process those images and so in that case
what will happen that system will not be
considered failed but the result after
deadline those results will not accept
it
like if a satellite has sent some images
at a particular time and that ground
station not able to process the whole
image so after some time that satellite
will again send the images so what this
ground station will do that ground
station will discard the previous images
and it will accept the new one so what
system will not be considered as field
but whatever result it's getting after
deadline those will not accept it and
the third one online ticket reservation
system so in the system we can say that
when we try to do any seed it will take
20 to 30 second now if that system is
taking more time let us say that system
actually took 40 second or 50 second and
after 50 second that system actually
booked seed successfully so here there
is no role of deadline here if the
deadline is missed system will not
consider it failed and after that
deadline as well if the reservation is
successful after 30 seconds so it
actually missed this deadline after that
deadline as well result accepted so we
are having three types of real-time
system so and these category actually
decided on the basis of this one whether
the results are accepted after deadline
or not and what are the consequences so
on the basis of these three we are
having three different type of real-time
system hard real-time system form
real-time system and
soft real-time system so hard real-time
systems says that if the system missed
its deadline that system will be
considered paint soft real-time systems
is that if the system missed its
deadline that system will not be
considered failed and result also
accepted and in case of firm real-time
system if system missed its deadline
system is not considered failed but late
results simply discarded so the example
we have seen airbag deployment comes
under hard real-time system satellite
tracking firm real-time system and
online reservation soft real-time system
thank you very much for watching