We have seen this slide earlier. This one tells the difference between the combinational logic and the sequential logic. And we mentioned about that key differences, the sequential circuit, there is a memory unit, which stores the current contents of the, the system. And this current content of the system will come back here, affect the combinational logic, which is going to eventually impact the outputs. So, the outputs is not only dependent on the input. It also depends on the memory units. So, before we talk about how to specify and how to design sequential systems, the first thing we need to, we need to answer is, so how do we store the memory unit? So the basic memory unit is called a latch, which has two outputs, a Q and a Q prime, which are always complementary to each other. And depends on what kind of latches you are using, there are different ways to fit the latches, and here is a small example of a NOR Gate for latch. So, what we have here is we have an input R and S stands for set and reset. And these are the two outputs, Q and Q complement. And in the middle there are two NOR Gates, and then they are cross coupled. So let's see, I mean, what happens with this circuit. So if I set S to be 0 and R to be, I'm sorry if I set S to be 1 and R to be 0. And this one is going to produce an output of 0 for gate two. Now then these two 0 will come back here. So now I have two 0s to NOR gate number 1. And that, it will produce output of 1. And this 1 comes back. And the two 1's make this NOR gate a constant 0. So what we see here is, the circuit becomes stable. And then we have Q equal to 1 and Q 0, Q prime equal to 0, complement to each other. So now what do I do is, now let's assume that I change the signal of S to 0. So instantaneously, this 0 is going to change. So this 0 and the 1 here makes this 1 still a 0 because I have a 1 here. 1 and 0 make this NOR gate a 0. So because this 0 doesn't change, for this gate, nothing's changed. 0 is here, and then 0 coming back, making a 1. So that means, if I change this to a 0, the system status doesn't change, 'kay? And you can analyze, once we further change this to a 1, other things will be different, because this 1, will change this to a 0, and then this 0, will, make this two 0s, change this one back to a 1. And this one coming back, make this one a stable 0. So the system will flip. The Q changes from 0, from 1 to 0. And then Q prime changes from 0 to 1. So what we have seen here is, so the output of this latch, Q and Q prime, depends on R and S. And however it can do becomes stabilized. Can store a value here, either 1 or 0. And one problem about this latch is the output changes as soon as the input changes. So, this is what we called asynchronous system and which may not be what we prefer. So, the flip-flops is a solution to solve this problem. It is very similar to a latch. But, however, it has a signal to control when output can change. So this is the RS Flip-Flop implemented using four NAND gates. And we're not going to go through details about how this one is implemented. We'll just show you the logical diagram for RS Flip-Flop. So what you see here is we have three inputs. SR, which we called the flip-flop input function. And there is this signal which is, normally, we call the cog pause. This is the cog signal. And for this systems, we want to make sure that at least one of the input our dies will be 0. We don't want to, to set and to reset at the same time, otherwise the system becomes unstable. And the systems have two output, Q and Q prime, which are supposed to be complement to each other all the time. And what will be the memory content? We'll assume that currently it has Q of t. And at time Q of t plus 1, what would this, this memory be? It is, depends on the input as an R, and also the current memory content following this formula here. S plus R prime times Q of t. And while we design sequential systems, we want to know that, how we can give, flip flop input functions values such that the flip flop would change as we want. So for example, consider this SR flip flop. Let's see, we want to change the Q from 0 to 1. How can we make that happen? So this table which is called excitation table tells you how you can make that happen. As soon as you set S equal to 1 and R equal to 0, you are going to set the flip flop, so next time the flip flop, it will have a 1. And to, for another example, if you want the system to stay at 1, 1, no change. What you do is you can set, reset. You code with 0, you're not going to reset. And then this set signal, you don't really care. And these are the excitation table for two of the three other popular flip flops. The D flip flop, flip flop, which the next content is the same as the data here, regardless of the previous value here. And then the toggle flip flop, which whenever the toggle signal is 1, the flip flop counted will be flipped. Otherwise it remains the same. And then the J K flip flop, which is similar to I's flip flop. So now let's use a small example to see how we can specify synchronous systems. We want to design a circuit with one input x and then three outputs, A, B, Cs. And the, the x will have one bit per clock cycle. When x equal to 0, the output remains no change. When x equal to 1, it's going to repeat the binary sequence 0, 1, 3, 7, 6, 4, one at a time. So what we have here is we want to draw the state transition table. So in the state transition table what we have is, we have a big column of the current state, and then we want to go from the current state to see what will be the next state. And then the next state depends on the input value, so based on different input values, the next stage can be different. Like in this case, if x is 0, the statement remains, no change. So that is why these two columns, they are identical. And then, while x is 1, we go from 0 to 1. We go from 1 to 3, and so on. And we can represent exactly the same information with this we called the state transition graph. In a state transition graph we have states, and for each state we have outgoing edges which are the transitions. It tells you that from this state, given an input where this next state will be, and what will be output. So, it represents exactly the same information as the state transition table. [SOUND]
