Flip-flops are memory elements that change state on clock signals. The major differences in these flip-flop types are the number of inputs they have and how they change state. There are basically four main types of latches and flip-flops: SR, D, JK, and T. After the rising or falling edge of the clock, the flip-flop content remains constant even if the input changes. This enable signal is usually the controlling clock signal. Flip-flops, on the other hand, have their content change only either at the rising or falling edge of the enable signal. In other words, when they are enabled, their content changes immediately when their inputs change. The main difference between latches and flip-flops is that for latches, their outputs are constantly affected by their inputs as long as the enable signal is asserted. One latch or flip-flop can store one bit of information. Latches and flip-flops are the basic elements for storing information. Using the signal Q as the state variable to describe the state of the circuit, we can say that the circuit has two stable states: Q= 0, and Q'= 1 hence the name “bistable.” Hence we can say that the bistable circuit can store one bit information in it so called as one bit memory cell. A bistable element has memory in the sense that it can remember the state of the circuit indefinitely. Similarly, if we start the circuit with Q= 1, we will get Q' = 0, and again we get a stable situation. A 1 going to the input of the top inverter will produce a 0 at the output Q, which is what we started off with. Since Q is also the input to the bottom inverter, Q', therefore, is a 1. However, Q will take on whatever value it happens to be when the circuit is first powered up.Īssume that Q = 0 when we switch on the power. Since the circuit has no inputs, we cannot change the values of Q and Q'. It has no inputs and two outputs labeled Q and Q'. The simplest sequential circuit or storage element is a bistable multivibrator, which is constructed with two inverters connected sequentially in a loop as shown in Figure below. The storage capability in sequential circuits is normally achieved by means of flip-flops. In this case, the system can be modeled as in Figure (b), where a feedback loop, containing the storage elements, can be observed. In contrast, a sequential logic circuit is one in which the outputs do depend on previous system states, so storage elements are necessary, as well as a clock signal that is responsible for controlling the system evolution. Thus the system is memory less and has no feedback loops, as in the model of Figure (a) below. Side note: yes, at one point in history computers were built out of relays.Definition: A sequential logic circuit is one whose outputs depend not only on its current inputs, but also on the past sequence of inputs.Ī combinational logic circuit is one in which the outputs depend solely on the current inputs.It's something you could reasonably test in the lab and it's pretty close to what you will likely see in practice. Putting combinational circuits together with sequential circuits makes finite state machines, which lead to more complex machines like computers.Ībstract ideas are powerful but can be hard to grasp without a specific example, that's why your coursework is focusing on SR latch as a specific example of a simple sequential circuit element. You're not studying a technology, you're studying the underlying design principles.Īrithmetic operations (such as unsigned 2's complement addition and subtraction) can be made from Combinational circuits (gates). Same basic design principles apply to all sequential circuits, regardless of the underlying technology. Could also be built from vacuum tubes or op-amps (with positive feedback and hysteresis) or some types of mechanical relays or even mechanical cogs and gears. A Set-Reset latch made of cross-coupled NOR is just one example of a way to realize a sequential circuit. "Sequential circuits" is an abstract idea. Step 1: understanding the problem statement sample input/output behavior: X: 0 0 1 0 1 0 1 0 0 1 0 Z: 0 0 0 1 0 1 0 1 0 0 0 X: 1 1 0 1 1 0 1 0 0 1 0 Z: 0 0 0 0 0 0 0 1 0 0 0 Finite string pattern recognizer (step 2) Step 2: draw state diagram for the strings that must be recognized, i.e.
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