 Now we shall discuss about sequential logic circuit. In sequential logic circuit there is a memory element, why it is used because the output of sequential logic circuit depends on the present input as well as also the past output. So let us see the block diagram of sequential logic circuit, combination logic circuit here it is input and here it is output and there is a memory element. So here there is a memory element which is connected to the combinational logic circuit to form a feedback path as the output of the sequential logic circuit depends on the present input as well as the past output. So what is this memory element, so memory element is a device which stores the binary information and when the memory element stores one bit of binary information means one bit that is called the flip flop or latches. Now we shall discuss about the different types of sequential logic circuit. So sequential logic circuits are of two types, one is synchronous sequential circuit and other is the asynchronous sequential logic circuit. So now what is synchronous logic circuit, the synchronous logic circuit is it depends on the clock pulse that means the clock pulse it is a signal which consists two states that means 0 and 1, this is a clock signal and in the clock pulse when there is a transition from 0 to 1 so this edge it is called leading edge or a positive edge that means there is a transition from 0 to 1 and when there is a transition from high to low that is 1 to 0 here 1 to 0 then this edge is called trailing or negative edge and the time during the transition of 0 to 1 then this time is called enabled and the condition from 1 to 0 that means high to low that is called the disabled. In case of synchronous sequential logic circuit the output is changes the state is changes during the transition either from 0 to 1 or 1 to 0 so that circuits are called sequential logic circuits. And next is the asynchronous logic circuits that means in asynchronous logic circuits there is a no clock pulse, clock signals and this asynchronous logic circuits it is a simply a combinational logic circuits with the feedback that means with the memory element. The output of the asynchronous circuit is depend upon the order in which its input signals change and the status can change at any instant of time and the asynchronous circuit do not use clock pulses and the change of the internal state occurs when there is a change in the input variable and asynchronous circuit is similar to the combinational circuits with feedback. Next is they are used when speed of operation is important now latches or flip flops latches and flip flops are one of the building blocks of sequential logic circuits and one latch can store one bit of information that is either 0 or 1 and only the difference between this latch and flip flop is that they are the method which is used to change the state. The output of latches are constantly affected by their inputs as long as the enable signal is asserted and a latch do not have a clock signal whereas a flip flop always does. So latches means it is a memory element it is a device which stores the one bit of information. Latches and flip flops are similar but the only difference is that the outputs are changes the the only difference is that the change of the state that means in case of latches the output is changes that means it stores two states either 1 or 0 one bit of information that means a latches can store 1 or 0 and it is changes the difference is that the output stages changes in case of latches the output is changes when the input is changes once the enable is asserted that means it is in the combinational logic circuit that it is a latches so this is a input output q focus a this q is status changes that means in latches the output status changes when a is changes that is input is changes as long as the enable the enable is asserted. So that is the difference but in case of flip flop the output change of the output stage is depend on the clock so that is the basic difference. Next we shall see the flip flop or it simply it can written as f f so flip flop is nothing but a memory element which can store one bit of information and this flip flop are generally are designed by using the NOR and NAND gates so let us first check the RS latches circuit using NOR gate so here R means it is expressed as reset and S is set so the logic circuit is for NOR gate using NOR gate this is a NOR gate one input this is another NOR gate this is one input so here R and other is the S and as there is a memory element is there that is a feedback part so this output of first upper NOR gate is goes to the input of lower NOR gate and similarly the output of the lower NOR gate will go to the input of the upper NOR gate. So there are two outputs in case of flip flop the two outputs are one is q and the other output is the complement of the first output that means if q is 0 the other output is will be 1 or if q is 1 the other output will be 0 that means the two outputs are complement of each other and if we see the truth table suppose R first is R then S one is q and that is q bar so these two are the inputs and these are two outputs so first we see the two conditions 0 0 then 0 1 1 0 1 1 so first for both the inputs that is R equal to S equal to 0 then what will be the output the output states q will be 1 and q will be 0 anyone so suppose we consider this q is 1 that means this is the first condition means 0 0 so q may be 1 and q may be 0 so it does not depends on the condition of the two inputs the NOR gates the output of the NOR gate does not depends on the condition of the two inputs that is R and S so the output of this combination that is 0 0 there is no change and C that means if you write here comment there is no change and for the second case when S is 1 that means S is 1 and R is 0 then what happened this output S is 1 so q bar q bar output is 1 is S plus this q so if as we know from the rule of NOR gate NOR gate what we know suppose 0 0 0 1 1 0 1 1 for OR gate what happened 1 1 1 and this is for OR gate and for NOR gate that is 1 0 0 0 just complement of the OR gate so what we have seen when any of the input is 1 the output is NOR gate output is 0 so S S is 1 in this case so output will be 0 S q bar is 0 that means q is what q will be 1 because q is the complement of q bar so for the second condition q is 1 q bar is 0 that means it is a set output because set when S is 1 that means set input is 1 the output will be the 1 next condition third condition what we have seen in the third condition R is 1 and S equal to 0 so what will be the q q is R plus q bar q bar is the output of the lower NOR gate so R is 1 that means definitely q is 0 according to this rule of the NOR gate so q is 0 means q is 0 that for therefore q bar is 1 so for the fourth case when S is 1 and R is 1 then what happened q will be 0 because q is R plus q bar okay then similarly q bar is also 0 because this S is 1 so any of the input is 1 output is 0 but this condition cannot be happened because we know that q is opposite of q bar that means if q is 1 the other output should be 0 but in this case we have got both the 0 so this condition is called R raise condition that means it is not used so it is called the raise condition so this we have discussed about the RS flip flop or legs by using the NOR gate so dear learner in this video class what we have learned first we have discussed about the subtractor so there are two types of subtractor one is half subtractor and the other is full subtractor and next we have discussed about sequential logic circuit so there are two types of sequential logic circuit one is synchronous and other is the asynchronous sequential circuit and we have also discussed about the latches and the flip flops then we have discussed about the RS latches that means by using a NOR gate in our next video class you will get the other flip flops like SR plugged flip flop JK flip flop master slap flip flop so these are all sequential logic circuits so we shall meet again in the next class thank you all