# Flip-flops

In this post I will explain what are flip-flops. Are memory digital or sequential circuits whose output depends of the input and the output`s actual state.

Latches

Are digital circuits with logic door NAND or NOR, where a logic door`s output is linked to another logic door`s input. This type of circuit can store one bit of information. This is a NAND latch. This circuit`s truth table. This is the NOR latch.  Clocks

Flip-flops need a square wave generator circuit called clock to work, an astable 555 can be a clock, click in the button below to know how to build one.

One alternative is the astable circuit with 7414. Other alternative is an astable with two transistors. Types of flip-flop

This is the SR flip-flop. In this circuit, the Q only change state when happen a positive transition of clock, by that, change from 0 (low level) state to 1 (high level) state or a negative transition (from level 1 to 0). Here is the truth table considering clock transitions. The circuit below is the JK flip-flop. In this circuit, there is no invalid response in any input condition. Note that the NAND doors have 3 inputs. Below we have the truth table considering clock transitions. The T flip-flop is a simplification of the JK, just link the J and K input. The truth table is also simplified.  This is the D flip-flop, the clock input can serve as an enabler. The D input only influence in the output with a positive or negativa clock transition. Here is this circuit’s truth table. Integrated circuits with flip-flop

Some integrated circuits to use flip-flop in practice.

• 74LS112 has 2 JK flip-flops;
• 74LS175 has 4 D flip-flops;
• CD4043 has 3 NOR latches and 3 NAND latches.

Exist many others integrated circuits with flip-flops and latches. The chips have terminals CLR (clear) and PR (preset), which are asynchronous inputs which can the flip-flop`s output independent of the clock and the synchronous inputs (J, K, S, R, D and T). 