Experiment - 1
Date of Performance: 29/06/2016 RollNo.:14
Aim:
Introduction to Advanced Digital System Design and VHDL
Objective:
Understanding the background of advanced digital system design and basic
concepts along with an introduction to VHDL technology.
Theory:
Digital vs. Analog: It is often said that we live in a digital world, but what does
that really mean? What is the opposite of or alternative to digital? In its simplest
form, a digital system is one that manipulates and stores binary values. A binary
value can have only one of two different values: TRUE or FALSE. The values
TRUE and FALSE are often represented by 1 and 0 respectively. We use the term
bit (short for binary digit) to refer to a single value consisting of either a 1 or a 0.
Using a single binary value you can only represent notions such as ON vs. OFF,
HIGH vs. LOW, HUNGRY vs. NOT-HUNGRY, etc. The physical representation
of a binary value in a digital system might be done using a voltage on a wire. In
this case a high voltage would correspond to a 1 (TRUE) and a low voltage
would correspond to a 0 (FALSE). The digital system would then contain circuits
to operate on those voltages (values) to do computations.
In contrast, an analog value is one that can take on any value from a continuous
range. An example of an analog quantity might be the representation of the
current temperature which could take on any value from the continuous range of -
120 to +120 degrees and would be mapped to the range of 0V to 2.4V. In this
case, each 0.01V change on the wire voltage might correspond to a 1 degree
change in temperature. Clearly, this quantity could represent the temperature
much more accurately than a single bit would be able to. The temperature could
(conceivably) take on any real number while in the digital case, the temperature
would be represented simply as one of two different quantities such as HOT vs.
NOT-HOT (COLD) where the dividing line between HOT and COLD might be
arbitrarily set at 80 degrees.
Digital representations of data have a number of benefits over analog
representations for storage, duplication, transmission, and processing. Does this
mean there is no longer a need for analog circuits? Absolutely not. The world we
live in is largely an analog world and analog techniques are needed to deal with
with it. That said, a general trend over the past few decades has been to
increasingly design systems which convert these analog quantities into equivalent
digital representations, and which then store and manipulate the resulting values
using digital rather than analog techniques. As will be shown in the remainder of
this text, digital circuits are easily designed and built to operate on digital (binary)
data and which can manipulate, store, and retrieve binary data.
Combinational vs. Sequential Digital Circuits: Another important use of digital
systems is for control circuits. For example, consider the creation of a machine to
control a car wash. The machine must react to the insertion of coins into its coin
box, it must sequence the car wash pumps in order to properly spray the car,
apply soap, rinse the car, etc. Further, it must time these various steps since each
takes a different amount of time. Finally, it must flash lights to communicate with
the user. A digital system is a good way of implementing such a controller.
Digital circuits can be divided into two broad categories — combinational
circuits and sequential circuits. Combinational circuits are the simpler of the two
and are circuits whose outputs are a simple and direct function of their inputs at
all times.
In contrast, a sequential digital system is one which possesses some form of
memory. At each time instant it makes a decision on what to generate for its
