Introduction:
HDL stands for Hardware Description Language. Many of us knew whats the use of HDL and how to design and model a digital circuit. But most of us doesnt know why actually this HDL is used,I mean many of us doesn't know the necessity of the HDL and why this HDL came into existence. In addition to these, most of the programmers assume that both HDL and programming language are same. But there exists some differences between them. And also one thing most of the books on HDL explains us the syntax of the various constructs present in the language, different styles of architecture writing, in depth explanation of the complicated concepts of HDL. But the answers for the above mentioned fundamental questions are not found in most of the books.
Necessity of the HDL:
Before going to necessity of the HDL, first we should know the concept of Modular Design. What the concept of modular design states is that a complex task can be divided into many small tasks called as Modules and finally the complex big task is achieved by integrating all the small modules together. This concept is the foundation on which whole of the HDL is developed.
Now we can think of the necessity of the HDL. Basically HDL is used to design the digital modules for Integrated Circuit(IC Chips). Hopefully as we all knew the structure of the circuit inside the IC chip is very complex. So designing that complex circuit structure in IC in one circuit is very tedious task. Assume we have designed such an complex circuit in one circuit, then debugging for errors in the circuit, if any found during simulation, is another difficult task. Future enhancements and any extensions for such type of circuits for sure it will be a nightmare for circuit designers. In order to make the work of circuit designers
simpler HDL is developed.
The main aim of the HDL is to make process of designing an circuit as much simpler as possible there by assisiting circuit designers. How HDL achieves this task is by using the approach of modular design. So basically what HDL does is that a complex big circuit is divided into many smaller circuits called Modules. After dividing, using HDL each module is assigned a smaller task, its input and output signals are defined. This process is done for all modules. Finally a hardware description of the entire is available which is used by the circuit designers to design the complex circuit in terms of small modules. Hence HDL is the standard way of describing the structure of a complex circuit.
Normally HDL is used in designing complex digital circuits like Integrated Circuit Chips, Digital Data Memory Devices, Motherboard and so on. Some of the available basic digital circuits includes universal gates, full and half adder, full subtractor, multiplexer, de-multiplexer, encoder, decoder, flip flops. So all the complex digital circuits can be realized in terms of the basic modules listed above. In other words we can say, any complex digital circuit can be divided into number of basic modules.
Consider the designing of a complex digital circuit such as 8085 processor, what HDL basically does is dividing the complex circuit of 8085 into basic digital modules (in other words, realizing the complex circuit of 8085 in terms of basic digital modules), defining the input and output signals (in other words task of each basic module is defined). After these, the main role of HDL is that the interconnection between these smaller modules are defined in such a way that the complex structure of the digital circuit is achieved. At last, the final HDL description of the digital circuit is used as a reference while designing the circuit.
Differences between HDL and programming language:
Finally coming to the difference between HDL and programming language, there are some small differences. Firstly, the HDL is extensively used in the pre-manufacturing stage of the ICs and the programming language is used on manufactured and tested ICs. Secondly the HDL is basically used to describe the hardware structure of the IC whereas the programming language is used to achieve a task from IC.
An Example:
Let
me take a simple example to explain the importance of HDL. Let us assume a Full
Adder and a 4-bit Parallel Adder. As we all know a full adder has three inputs
and one output. It takes two data bits and one carry input bit as input data,
adds them and generates one sum bit and one carry out bit as output bits. The
block diagram of a full adder is as shown below:
Figure: Block diagram of a full
adder
The data bits are a,b and carry
input bit is Cin. Cout is the carry out bit. Full adder adds the bits ‘a’ and
‘b’ along with ‘Cin’ and generates ‘Sum’ and ‘Cout’ as output bits. Hence the
hardware description of a Full adder consists of three input signals and two
output signal.
The truth table of Full adder is:
a
|
b
|
Cin
|
Sum
|
Cout
|
0
|
0
|
0
|
0
|
0
|
0
|
0
|
1
|
1
|
0
|
0
|
1
|
0
|
1
|
0
|
0
|
1
|
1
|
0
|
1
|
1
|
0
|
0
|
1
|
0
|
1
|
0
|
1
|
0
|
1
|
1
|
1
|
0
|
0
|
1
|
1
|
1
|
1
|
1
|
1
|
Looking at the truth table of
Full adder, the equation for Sum is
Sum = (a.(b`).(Cin`)) + ((a`).b.(Cin`)) + ((a`).(b`).Cin)
+ (a.b.Cin) = (a xor b) xor Cin
And the equation for Cout is
Cout = (a.b) + (b.Cin) + (Cin.a)
Where
. is Logical AND operation
+ is Logical OR operation
` is the Logical NOT operation
The Structure of a
Full Adder is:
Figure: Structure of a Full Adder
Now consider a 4-bit parallel
adder, whose block diagram is as shown below:
Figure: Block diagram of a 4-bit
Parallel adder
Basically what a 4-bit Parallel
adder does is it two sets of 4-bit data all at one time, adds them and
generates a 4-bit sum and a carry out at one shot. Hence the name parallel
adder. An overview of how an 4-bit parallel adder works is as given below: let
a(0-3) and b(0-3) be 4-bit data set and S(0-3) be the 4-bit sum and Cout be the
carry out.
a(0) + b(0) à S(0), c(1)
a(1) + b(1)+c(1) à S(1), c(2)
a(2) + b(2) + c(2) à S(2), c(3)
a(3) + b(3) + c(3) à S(3), c4 = Cout.
The meaning of the above
equations are to generate a sum bit and a carry out bit, the carry out
generated in the additon of previous bit is taken as carry input of next stage.
For the initial bit, the carry input is taken as zero.
By comparing the functionalities
of a Full adder and 4-bit Parallel adder, the function of the latter is quite
complex and so is the structure of the circuit. Assuming that 4-bit parallel
adder is realized in one circuit model, its a quite complicated task and the
structure of such a circuit is also quite complex. Hence we need to make the
complex circuit of 4-bit parallel adder as much simpler as possible. Looking at
the functionality of the 4-bit parallel adder, it can be toought four full adders connected in cascade. Now
the role of an HDL here is to define the input and output signals for each full
adder and also to define the interconnection patterns between the full adders
in a standard way inorder to achieve the reqired function of 4-bit full adder.
This hardware description is used by the circuit designers, who comes next in
the process of IC manufacturing, to design the complex circuits in simpler
manner.
The simpler structure of 4-bit
parallel adder is:
Figure: Simpler structure of a
4-bit Parallel adder
The simpler structure of 4-bit parallel
adder consists of four full adders connected in cascade. The hardware
description of 4-bit parallel adder consists of two sets of 4-bit data, a(0-3)
and b(0-3), a carry input bit Cin as input signals and 4-bit sum s(0-3), carry out Cout as output signals. In
addition to these we have three signals c(1-3), intermediate carry, as
interconnecting signals, which are both input signal to the next bit addition
and output signal of the previous bit addition.
One thing we should remember that
as the number functions performed by the circuit increases, it is quite obvious
that the complexity of the circuit also increases. So don’t think of necessity
of HDL to small circuits like 4-bit parallel circuit. I have given this simple
example just to make you to understand that as the number functions performed
by the circuit increases, the complexity of the circuit also increases. But if
we think of designing multiple tasks performing cicuits like processor chips,
microcontroller chips, digital memory devices such as RAM, Hard disks so on,
motherboard circuits, definitely we find the necessity of HDL since structure
of these circuits are quite complicated. And also one thing if the structure of
the digital circuit is simpler, we will not find much the necessity of HDL.
Basically HDL is developed to make the process of designing circuits which are
having highly complicated structures easier.
