What is the Hierarchy of a Digital System

A digital system has many advantages over an analog system such as cost, speed of operation, sophistication of processing, reliability etc. Therefore in real world systems, most of the signal processing is done in the digital domain.

In the modern age, a person comes across many digital systems in his or her everyday life. Computers, digital clocks, digital instruments (such as Multimeters), control systems (such as traffic light controllers, flow and washing cycle controllers in washing machines, ignition control in cars), audio and video processing systems ( such as in television and digital radio) etc. are all examples of digital systems.

Then what is meant by the hierarchy of a digital system. From a designer’s point of view, any digital system no matter how much complex can be broken down in to smaller and smaller units using a ‘top down approach’ until the device level implementation of the circuit can be synthesized. Let us first of all elaborate how this hierarchy works and then try and clarify the same with the use of an example of a Personal Computer.

A complex digital system can first of all be broken down in to a number of sub systems that are working in collaboration with each other to form the entire system. Each of these sub systems can then be broken down in to different modules or functional units. These functional units are synthesized from the basic digital units that comprises of combinations of various gates. This gate level implementation can further be broken down then in to actual circuits consisting of active and passive circuit elements.

The hierarchy discussed above can be better explained by using Personal Computer as an example. A personal computer being a digital system can first of all be broken down in to sub systems such as a Central Processing Unit, Memory, I/O systems etc. Using the example of CPU as a sub system, it can then be broken down in to functional modules such as an arithmetic unit, a logical unit, control unit, registers etc. Let us say the arithmetic unit is to be designed first of all. It is common knowledge that it can be constructed by combining an adder circuit, a subtraction circuit, multiplication circuit and a division circuit. All these circuits can be synthesized by using basic gates like AND, OR, NOT etc in various combinations. We all know that these gates are nothing but circuit elements like transistors, resistors, capacitors, diodes etc. in various circuit arrangements. Thus a complex system like a digital computer is broken down to its device level implementation by using a ‘top down’ hierarchal approach.

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