Test: Flip Flops

Latch is a type of bistable multivibrator having two stable states. Both inputs of a latch are directly connected to the other’s output. Such types of structure is called cross coupling and due to which latches remain in the latched condition.

The SR flip-flop actually has three inputs, Set, Reset and its current state. The Invalid or Undefined State occurs at both S and R being at 1.

In D flip flop, when the clock is high then the output depends on the input otherwise reminds previous output. In a state of clock high, when D is high the output Q also high, if D is ‘0’ then output is also zero. Like SR flip-flop, the D-flip-flop also have an invalid state at both inputs being 1.

In sequential circuits, the output signals are fed back to the input side. So, The circuits whose outputs at any instant of time depends only on the present input but also on the past outputs are called sequential circuits. Unlike sequential circuits, if output depends only on the present state, then it’s known as combinational circuits.

There are two type of sequential circuits viz., (i) synchronous or clocked and (ii) asynchronous or unclocked. Synchronous Sequential Circuits are triggered in the presence of a clock signal, whereas, Asynchronous Sequential Circuits function in the absence of a clock signal.

The basic latch consists of two inverters. It is in the sense that if the output Q = 0 then the second output Q’ = 1 and vice versa.

The output of latches will remain in set/reset untill the trigger pulse is given to change the state.

The cross-coupled connections from the output of one gate to the input of the other gate constitute a feedback path. For this reason, the circuits of NOR based S-R latch classified as asynchronous sequential circuits. Moreover, they are referred to as asynchronous because they function in the absence of a clock pulse.

The SR flip-flop is very effective in removing the effects of switch bounce, which is the unwanted noise caused during the switching of electronic devices.

After one cycle the value of each input comes to the same value.
Eg: Assume J = 0 and K = 1. After 1 cycle, it becomes as J = 0 → 1 → 0(1 cycle complete) and K = 1 → 0 → 1(1 cycle complete).
The J & K flip-flop has 4 stable states: Latch, Reset, Set and Toggle.

The basic S-R flip-flop can be constructed by cross coupling of NOR or NAND gates. Cross coupling means the output of second gate is fed to the input of first gate and vice-versa.

Combinational circuits are often faster than sequential circuits. Since the combinational circuits do not require memory elements whereas the sequential circuits need memory devices to perform their operations in sequence. Latches and Flip-flops come under sequential circuits.

The sequential circuit is also called a latch because both are a memory cell, which are capable of storing one bit of information.

In S-R flip-flop, if Q = 0 the output is said to be reset and set for Q = 1.

Trigger pulse is defined as a pulse that starts a cycle of operation.