Digital Logic Levels
Logic level A voltage level that represents a defined digital state in an electronic circuit.
Logic HIGH (or logic 1) The higher of two voltages in a digital system with two logic levels.
Logic LOW (or logic 0) The lower of two voltages in a digital system with two logic levels.
Positive logic A system in which logic LOW represents binary digit 0 and logic HIGH represents binary digit 1.
Negative logic A system in which logic LOW represents binary digit 1 and logic HIGH represents binary digit 0.
When we want to describe a digital quantity electronically, we need to have a system that uses voltages or currents to symbolize binary numbers.
The binary number system has only two digits, 0 and 1. Each of these digits can be denoted by a different voltage called a logic level. For a system having two logic levels, the lower voltage (usually 0 volts) is called a logic LOW or logic 0 and represents the digit 0.
The higher voltage (traditionally 5 V, but in some systems a specific value such as 1.8 V, 2.5 V or 3.3 V) is called a logic HIGH or logic 1,which symbolizes the digit 1. Except for some allowable tolerance, as shown in Figure 1.2, the range of voltages between HIGH and LOW logic level is undefined.
FIGURE 1.2
Logic Levels based on 5 V and 0 V
For the voltages in Figure 1.2:
5 V = Logic HIGH = 1
0 V = Logic LOW = 0