1EC262 Topic 2: Introduction to Combinational Systems Much of a computer’s hardware is comprised of digital logic circuits. Digital logic circuits are built from just a handful of primitive elements, called logic gates, combined in various ways. In a digital logic circuit, only two values may be present. The values may be 5−and + 5 volts. Or the values may be 0.5 and 3.5 volts. Or the values may be… you get the picture. To allow consideration of all of these possibilities, we will say that digital logic circuits allow the presence of two logical values: 0 and 1. So, signals in a digital logic circuit take on the values of 0 or 1. Logic gates are devices which compute functions of these binary signals. The Three Basic Logic Gates AND Gate. Consider the circuit below which has 2 switches in series: Switch S1Switch S2LightWhen will the light turn on? We can represent the light's operation using a table: S1 S2 Light open open off open shut off shut open off shut shut on Change the words openand offto 0and the words shutand onto 1and the table becomes S1 S2 Light This is the truth tablefor the AND operation. 0 0 0 0 1 0 1 0 0 1 1 1 The simplest AND gate gas two inputs and one output. If both inputs are 1then the output is 1. Otherwise, the output is 0. The symbol for the AND gate is: We represent the AND operation by using either the multiplication symbol (i.e., “ ∙“) or by writing the inputs together. Thus, for the AND gate above, we would write the output cas ca b=or as c= ab. This would be pronounced: “C= aand b.”
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