A Primer on Digital Logic Part-II

[Music] Let's now take a look at sequential logic. So sequential logic is slightly different from combinational logic. So here the idea is that we save a value in a structure and uh then uh we do some amount of computation on it. Uh so it's called a logic cloud, right? We have a lot of gates and so on.

and then again uh the output of this is again stored in another storage element. So the first question that we need to answer is that how we how do we store a single bit right how do we ensure that we are able to store a single bit and uh the the the value sort

of remains for as long as we want to keep it. So the idea is that to store a bit we use a cross-oupled pair of NAND gates and that will store a stable value. So this particular structure over here is called an SR latch and so S stands for set. So what does set mean? uh set means

set the value the the value of the output which in this case is Q to one and R means reset. Reset means set Q to zero. So the SR latch has two inputs uh which is basically the complement of S and the complement of R and the output and there are two outputs one output is

Q and the other is the complement of Q. So in a in a sense it's a single output but available in its normal form as well as its complemented form as simple as that. And why do we say it is crossoupled? The reason being that the output of the first nan gate is an input

to the second nan gate. Right? Let's maybe call this n_sub_1 and let's call it n_sub_2. So the output of n_sub_1 is an input to n_sub_2 and the output of n_sub_2 is an input to n_sub_1. So that's the reason we call it a cross-oupled nan gate. So let's see how it works. The way that it works is