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Subject: Introduction
Date: Sat, 1 Jan 2011 14:47:47 +0200
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<P><BIG><BIG><BIG><STRONG>Introduction</STRONG></BIG></BIG></BIG></P>
<P><FONT face=3DArial>Combinational logic refers to circuits whose =
output is=20
strictly depended on the present value of the inputs. As soon as inputs =
are=20
changed, the information about the previous inputs is lost, that is,=20
combinational logics circuits have no memory. In many applications, =
information=20
regarding input values at a certain instant of time is required at some =
future=20
time. Although every digital system is likely to have combinational =
circuits,=20
most systems encountered in practice also include memory elements, which =
require=20
that the system be described in terms of sequential logic. Circuits =
whose=20
outputs depends not only on the present input value but also the past =
input=20
value are known as <STRONG>sequential logic circuits</STRONG>. The =
mathematical=20
model of a sequential circuit is usually referred to as a =
<STRONG>sequential=20
machine</STRONG>.</FONT></P>
<P><FONT face=3DArial>A general block diagram of a sequential circuit is =
shown=20
below in Figure 1.</FONT></P>
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<P align=3Dcenter><BIG>Figure 1. Block Diagram of Sequential =
Circuit.</BIG></P>
<P align=3Dleft><FONT face=3DArial>The diagram consists of combinational =
circuit to=20
which memory elements are connected to form a feedback path. The memory =
elements=20
are devices capable of storing binary information within them. The =
combinational=20
part of the circuit receives two sets of input signals: one is=20
<STRONG>primary</STRONG> (coming from the circuit environment) and=20
<STRONG>secondary</STRONG> (coming from memory elements). The particular =

combination of secondary input variables at a given time is called the=20
<STRONG>present state</STRONG> of the circuit. The secondary input =
variables are=20
also know as the <STRONG>state variables</STRONG>.</FONT></P>
<P align=3Dleft><FONT face=3DArial>The block diagram shows that the =
external outputs=20
in a sequential circuit are a function not only of external inputs but =
also of=20
the present state of the memory elements. The next state of the memory =
elements=20
is also a function of external inputs and the present state. Thus a =
sequential=20
circuit is specified by a time sequence of inputs, outputs, and internal =

states.</FONT></P>
<P><A href=3D"javascript:loadSession('quiz1.htm')"><BIG><STRONG>Short=20
Quiz</STRONG></BIG></A></P>
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