Page 47 - CITS - Electronic Mechanic - TT - 2024

ELECTRONICS MECHANIC - CITS

Here is the difference between synchronous and asynchronous sequential circuits

Synchronous Sequential Circuit: Output changes at discrete interval of time. It is a circuit based on an equal state
time or a state time defined by external means such as clock. Examples of synchronous sequential circuit are Flip
Flops, Synchronous Counter.

Asynchronous Sequential Circuit: Output can be changed at any instant of time by changing the input. It is a circuit
whose state time depends solely upon the internal logic circuit delays. Example of asynchronous sequential circuit
is Asynchronous Counter.
Basic Flip Flops
A circuit that changes from 1 to O or from O to 1 when current is applied. It is one bit storage location.
Flip flops are actually an application of logic gates. When a certain input value is given to them, they will be re-
membered and executed, if the logic gates are designed correctly. A higher application of flip flops is helpful in
designing better electronic circuits.

The most commonly used application of flip flops is in the implementation of a feedback circuit. As a memory relies
on the feedback concept, flip flops can be used to design it.
Latches and flip-flops are the basic elements for storing information. One latch or flip flop can store one bit of
information. The main difference between latches and flip-flops is that for latches, their outputs are constantly af-
fected by their inputs as long as the enable signal is asserted. In other words, when they are enabled, their content
changes immediately when their inputs change. Flip-flops, on the other hand, have their content change only either
at the rising or falling edge of the enable signal. This enable signal is usually the controlling clock signal. After the
rising or falling edge of the clock, the flip-flop content remains constant even if the input changes.
There are basically four main types of latches and flip-flops: SR, D, JK, and T. The major differences in these flip-
flop types are the number of inputs they have and how they change state. For each type, there are also different
variations that enhance their operations.
RS Latch
• RS latch have two inputs, S and R. S is called set and R is called reset.
• The S input is used to produce HIGH on Q ( i.e. store binary 1 in flip-flop).

• The R input is used to produce LOW on Q (i.e. store binary O in flip-flop).
• Q’ is Q complementary output, so it always holds the opposite value of Q.
• The output of the S-R latch depends on current as well as previous inputs or state, and its state (value stored)
can change as soon as its inputs change.
There are mainly four types of flip flops that are used in electronic circuits
1 The basic Flip Flop or S-R Flip Flop
2 Delay Flip Flop [D Flip Flop]

3 J-K Flip Flop
4 T Flip Flop
1 SR Flip Flop
The SET-RESET flip flop is not designed with the help of two NOR gates and also two NAND gates. These flip
flops are also called S-R Latch.
S-R Flip Flop using NOR Gate
The design of such a flip flop includes two inputs, called the SET [S] and RESET [R].
There are also two outputs, Q and Q’. The diagram and truth table is shown below.
The operation has to be analyzed with the 4 inputs combinations together with the 2 possible previous states.

From the diagram it is evident that the flip flop has mainly four states. They are

32
CITS : E & H - Electronics Mechanic - Lesson 9 - 29