in this presentation we will study about the sequential circuits a very very important presentation in the digital electronics course because this is the base for your flip flops counters and registers all these three topics very important and there are so many questions being asked from this three topics only so if you want to master these three topics you have to understand what is sequential circuits first that we will study in this presentation only there is also a small change that you will find in the coming videos and this presentation is the lecture number I'm mentioning
the lecture number like this is 118th lecture in the digital electronics course this will help the students who are learning the digital electronics first time okay so let's move to the sequential circuits what it is when we talk about the sequential circuits it becomes important for us to talk about the combinational circuits at the same time okay because half of the definition of the sequential circuit is the combinational circuit let's see how this is a formal definition for your sequential circuit you can use this definition if someone ask you what is sequential circuit in your
exams as well so in sequential circuits the present output okay the first important thing is the present output depends upon the present input okay now this two things when combined together makes your combinational circuit in combinational circuit the present output depends only on the present input okay we have to use word only here for the combinational circuit because if I say I'm having a combinational circuit that takes takes two input the first input is let's say one and the second input is also one and the combinational circuit here here adds this two numbers so 1
+ 1 gives you zero as the sum and one as the carry okay so this is the result that we are having again you are using this same same adder and you want to add zero and one then the result will be what 1 0er it has nothing to do with this output okay what was the previous output let me explain you once more you are adding one one okay so you are getting the result 01 this is your output okay now again you are adding different number let's say 0 1 then the output is
1 Z now this output this is your present output that we are talking about this present output is only depending upon this present input this is your present input this this one is your past input and this one is your past output fine so this output the present output has nothing to do with the past input or the past output thus we call it as the combinational circuit okay I hope you are getting this but in case of sequential circuit the present output depends upon the present input as well as as well as it depends
upon the past output or outputs okay like in this case we are having two outputs so it depends upon the past output or outputs as well so this is the major difference between your sequential circuit and the combinational circuit let's see how it works for example this is my combinational circuit this is my input present input and this is my present output so the present output depends only on the present input now if I say I want to convert this combinational circuit to the sequential circuit then what I have to do let's see I have
to take this output the all the three outputs actually and then store it I have to store it now let me explain you why I need to store it counter is the sequential circuit and you already know that how counter Works let's say if it is the decimal counter it counts from 0 to 9 every time it adds 1 to 0er then we will get 1 then again it adds 1 to 1 we get two then 2 + 1 3 then 4 5 6 7 8 9 so every time it is adding adding one to
the previous output because 0 + 1 gives us one now when you add one to one we must know that this one was was the previous output because until and unless you know what was the previous output how you going to add one for example if I'm at five then the counter will count six it will count six only if it knows that the previous output was five until and unless it knows what was the previous output how it is going to add one so we require to store we required to store previous output this
is the very important point in the sequential circuit we require to store the previous output so what we need we need a memory okay so what I'm actually doing here I'm giving inputs I'm getting the output and also at the same time and storing that particular output or outputs in my memory block we will talk about this memory very important point this memory will keep the output till the time I'm again going to use it let's say time came and we have to use the values stored in this memory and these values will be used
as the inputs so let's see what we got here we are having three inputs these are our present input then we are having the output and also we are having the memory element which has already stored the output from the previous inputs so this will become our past output and this past output will go to the input and you can call it as the feedback we go as the feedback and the present output will depend upon this present input and the past output so this is our sequential circuit now let me explain you the important
Point what is this map memory okay because in sequential circuit everything is combinational you can see everything is combinational except this memory and the feedback so we have to focus on this memory that what is this memory and how we are going to store the output and as we are in the digital world the output will be Zer or one bits you can say so we have to store the bits how we are going to do that if we know how to store a single bit we later can store multi- bits also so the first
thing that came in the mind of the people is to use cascaded not gate okay before that let me tell you this memory is nothing but your flip flops okay I have raised the curtains the suspense is over this memory is your flip flops okay but people didn't get to the flip-flops directly but they have thought different methods to store a single bit number earlier it was the cascaded not gate Arrangement that can be used to store a single bit okay let's see how these are your two not Gates and if the input is one
or high the output of your first not gate is zero and then the output of your second not gate is one so you can see that this one is stored
