How does latch work

Preferably, both Logic gates are equal and the device will be in an undefined condition for an indefinite stage. The simple extension of an SR latch is nothing but a gated SR latch.

It gives an Enable line that should be driven high before information can be latched. Although a control line is necessary, the latch is not synchronous due to the inputs which can alter the output even in the middle of an enable pulse. As the enable line is stated, a gated SR-latch is equal in the process toward an SR latch. The data latch is an easy expansion to the gated SR-latch that eliminates the chance of unacceptable states of input. The D-latch outputs the input of the D when the Enable line is high, otherwise, the output is whatever the D input was whenever the Enable input was last high.

This is the reason it is known as a transparent latch. A gated D latch is designed simply by changing a gated SR-latch, and the only change in the gated SR-latch is that the input R must be modified to inverted S.

The circuit of the latch will not at all experience a Race state due to the only D input is reversed to offer to both the inputs. Therefore, there is no possibility for similar input state.

Thus the circuit of D-latch can be securely used in several circuits. The both JK latch , as well as RS latch, is similar. It is an unwanted situation that occurs when a device attempts to perform two operations at the same time i. There are few ways in which we can avoid race around condition like using Edge Triggering or by using Master Slave Flip — flop. State table is similar to truth table in combinational circuits that gives the information about the states of a circuit.

As the outputs of sequential circuits depend on present and previous states, these are represented in the form of table called state table and it shows the next state based on the present state and other inputs. In addition to tables and equations, a state machine or a system can be represented by a state diagram.

In this state diagram, a state is represented by a circle, and the transition between states is represented by lines or arcs that connect the circles. State diagram for a simple SR latch is shown below. The state diagram provides all the information that a state table can have.

This is obtained from the state table directly. Generally, latches are transparent i. This can be achieved with the use of an extra input enable or clock or gate. If the enable or clock or gate signal is not asserted, the inputs are ignored and the outputs are latched to the previous values. In order to use this extra signal, additional logic should be added. These circuits are called Gated or Clocked Latches. When enable or clock is high, the latch is said to be enabled i.

When enable or clock is low, the latch is disabled and remains in that state until enable is asserted. Electrical Engineering Stack Exchange is a question and answer site for electronics and electrical engineering professionals, students, and enthusiasts.

It only takes a minute to sign up. Connect and share knowledge within a single location that is structured and easy to search. At first, I have to say, I'm not in Electrical Engineering, I'm studying for Computer Fundamental, and this question popped up, after a while considering, I decided to post it here. I found some "quite similar" topics when posting this, but can't understand at all. Maybe because the OP and the answer providers are talking things like electrical engineers!

Which of the following is the correct truth table for the flip-flop? This is the truth table I found on the internet, which indicates a is the right answer:. And how can we determine that Q will be "no change" or "unstable"? I believe there is a clear explanation for people like me can understand it, not only engineers!

They latch their outputs due to the interconnected gates, as you see in the first diagram. Nothing happens to the output when the inputs are not changed. The truth table of the NOR gate is important because it shows how the two parts of the SR Flip Flop interact - the NOR gate's outputs are fed into each other's inputs, which gives you the latching effect of the output. You can put both S and R inputs HIGH at the same time if you wanted, but it does not form for to the digital theory of "Q and NOT Q" outputs, so it's not normally acceptable and is called "illegal" in the truth tables.

Bad things happen with both inputs are set low, if both inputs were previously high, because of the gate delays of the NOR gates. This can cause oscillations of the output due to the feedback in the circuit. You can read more about these race conditions from here and here. Looking at the graphic, below, the lower image shows the logic symbol for a conventional positive true NOR gate, and the upper image shows the symbol for its so-called DeMorgan equivalent, a negative true AND.

The truth table shows them to be equivalent, logically, and the easy way - for me - to relate [to] them is to consider the straight line input part of the upper gate to denote "AND", and the curvy line of the lower one to denote "OR".

That way, since the upper gate is an AND and is shown shown with two bubbles as inputs, it reads: "two zeros make a one", and the lower gate, being a NOR, reads: "any one makes a zero". Where "A" is the basic latch and where red indicates a logic high and blue indicates a logic low. Referring to "B" and perusing the NOR's truth table, we find that if R is high a logic 1 then, regardless of the state of U, Q must be low. The same is true for U2, with the result being that if R and S are both held high, Q and notQ must remain forced low - and are therefore stable - until either R, or S, or both change state.

In "D", we now force U low while leaving U high, which will drive Q high and SET the latch, and since the inputs of U1 are now both lows, its output will go high and force U high assuring the latch will stay SET no matter what U does. It's important to notice that with "R" and "S" both low and the latch SET, the latch is stable and in one of its quiescent states.

You are right in believing that there should be an easier explanation that makes more sense to non-engineer people. The trick lies in moving away from truth-tables through approaching logic gates from their physical component: transistor—electricity controlled switches. Apply high voltage to it, it will become a conductor, apply low voltage to it, it is an insulator. The trouble with the latch structure is, two logic gates are feeding into each other, so there are only two externally controllable inputs.

Hence if you start from the truth-table, you will end up with a chicken-and-egg problem: to know the input, you have to know the output which comes from the input….