In our previous sections, we learned about combinational circuit and their working. The combinational circuits have set of outputs, which depends only on the present combination of inputs. Below is the block diagram of the synchronous logic circuit.
The sequential circuit is a special type of circuit that has a series of inputs and outputs. The outputs of the sequential circuits depend on both the combination of present inputs and previous outputs. The previous output is treated as the present state. So, the sequential circuit contains the combinational circuit and its memory storage elements. A sequential circuit doesn’t need to always contain a combinational circuit. So, the sequential circuit can contain only the memory element.
Difference between the combinational circuits and sequential circuits are given below:
|Combinational Circuits||Sequential Circuits|
|1)||The outputs of the combinational circuit depend only on the present inputs.||The outputs of the sequential circuits depend on both present inputs and present state(previous output).|
|2)||The feedback path is not present in the combinational circuit.||The feedback path is present in the sequential circuits.|
|3)||In combinational circuits, memory elements are not required.||In the sequential circuit, memory elements play an important role and require.|
|4)||The clock signal is not required for combinational circuits.||The clock signal is required for sequential circuits.|
|5)||The combinational circuit is simple to design.||It is not simple to design a sequential circuit.|
Types of Sequential Circuits
Asynchronous sequential circuits
The clock signals are not used by the Asynchronous sequential circuits. The asynchronous circuit is operated through the pulses. So, the changes in the input can change the state of the circuit. The asynchronous circuits do not use clock pulses. The internal state is changed when the input variable is changed. The un-clocked flip-flops or time-delayed are the memory elements of asynchronous sequential circuits. The asynchronous sequential circuit is similar to the combinational circuits with feedback.
Synchronous sequential circuits
In synchronous sequential circuits, synchronization of the memory element’s state is done by the clock signal. The output is stored in either flip-flops or latches(memory devices). The synchronization of the outputs is done with either only negative edges of the clock signal or only positive edges.
Clock Signal and Triggering
A clock signal is a periodic signal in which ON time and OFF time need not be the same. When ON time and OFF time of the clock signal are the same, a square wave is used to represent the clock signal. Below is a diagram which represents the clock signal:
A clock signal is considered as the square wave. Sometimes, the signal stays at logic, either high 5V or low 0V, to an equal amount of time. It repeats with a certain time period, which will be equal to twice the ‘ON time’ or ‘OFF time’.
Types of Triggering
These are two types of triggering in sequential circuits:
The logic High and logic Low are the two levels in the clock signal. In level triggering, when the clock pulse is at a particular level, only then the circuit is activated. There are the following types of level triggering:
Positive level triggering
In a positive level triggering, the signal with Logic High occurs. So, in this triggering, the circuit is operated with such type of clock signal. Below is the diagram of positive level triggering:
Negative level triggering
In negative level triggering, the signal with Logic Low occurs. So, in this triggering, the circuit is operated with such type of clock signal. Below is the diagram of Negative level triggering:
In clock signal of edge triggering, two types of transitions occur, i.e., transition either from Logic Low to Logic High or Logic High to Logic Low.
Based on the transitions of the clock signal, there are the following types of edge triggering:
Positive edge triggering
The transition from Logic Low to Logic High occurs in the clock signal of positive edge triggering. So, in positive edge triggering, the circuit is operated with such type of clock signal. The diagram of positive edge triggering is given below.
Negative edge triggering
The transition from Logic High to Logic low occurs in the clock signal of negative edge triggering. So, in negative edge triggering, the circuit is operated with such type of clock signal. The diagram of negative edge triggering is given below.