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Digital circuits are electric circuits based on a number of discrete voltage levels. In most cases there are two voltage levels: one near to zero volts and one at a higher level depending on the supply voltage in use. These two levels are often represented as L and H. 1 Levels
The two levels are used to represent the binary integers or logic levels of 0 and 1. In active-high logic, L represents binary 0 and H represents binary 1. Active-low logic uses the reverse representation. It is usual to allow some tolerance in the voltage levels used; for example, 0 to 2 volts might represent logic 0, and 3 to 5 volts logic 1. A voltage of 2 to 3 volts would be invalid and would occur only in a fault condition or during a logic level transition, as most circuits are not purely resistive, and therefore cannot instantly change voltage levels. However, few logic circuits can detect such a fault, and most will just choose to interpret the signal randomly as either a 0 or a 1.
Examples of binary logic levels:
| Technology | L voltage | H voltage | Notes |
|---|
| CMOS | 0V to VCC/2 | VCC/2 to VCC | VCC = supply voltage |
| TTL | 0V to 0.8V | 2V to VCC | VCC is 4.75V to 5.25V |
2 Construction
It is possible to construct digital circuits in forms other than electronic. In principle, any technology capable of representing two discrete states and performing Boolean operations could be used to build a logic circuit. Hydraulic, pneumatic and mechanical versions of logic gates exist and are used in situations where electricity cannot be used. The first two types are considered under the heading of fluidics. One application of fluidic logic is in military hardware that is likely to be exposed to a nuclear electromagnetic pulse (nuclear EMP, or NEMP) that would destroy any electrical circuits.
3 Logic systems
Logic systems can be constructed from diverse systems including optical, magnetic, chemical, biochemical and quantum systems. In each case, the desired logic function can be found in the interactions of the physical components. For example if two particular enzymes are required to prevent the construction of a particular protein, this is the equivalent of a biological "NAND" gate.
Digital circuits are the most common hardware realisation of Boolean algebra and are the basis of all digital computers. (See also logic gate.)
They can also be used to process digital information without being connected up as a computer. Such circuits are referred to as "random logic".
4 Recent developments
The discovery of superconductivity has enabled the development of Rapid Single Flux Quantum (RSFQ) circuit technology, which uses Josephson junctions instead of transistors. Most recently, attempts are being made to construct purely optical computing systems capable of processing digital information using nonlinear optical elements.
5 See also
Analog circuit |
Boolean algebra |
CircuitThere are many kinds of circuit An electric circuit interconnects electrical elements. Examples in electronics include printed circuits integrated circuits digital circuits Series and parallel circuits Telecommunication circuits A sport circuit is a close |
CMOS |
Combinatorial logicThis article is not about combinatory logic, a topic in mathematical logic. In digital circuit theory, combinatorial logic (also called combinational logic is a type of logic circuit whose output is a function of only the present input. This is in contras |
Data strobe encodingData strobe encoding and recovered clock. Data strobe encoding (or D/S encoding) is an encoding scheme for transmitting data in digital circuits. It uses two signal lines (e. wires in a cable or traces on a printed circuit board), Data and Strobe''. These |
De Morgan's lawsIn logic, De Morgan's laws (or De Morgan's theorem), named for nineteenth century logician and mathematician Augustus De Morgan, are the two rules of propositional logic, boolean algebra and set theory :not (P and Q) (not P) or (not Q) :not (P or Q) (not |
DigitalA digital system is one that uses discrete values rather than a continuous spectrum of values: compare analog. The word comes from the same source as the word digit: the Latin word for finger (counting on the fingers) as these are used for discrete counti |
Electrical networkAn electrical network or electrical circuit is an interconnection of analog electrical elements such as resistors, inductors, capacitors, diodes, switches and transistors. It can be as small as an integrated circuit on a silicon chip, or as large as an el |
ElectronicsElectronics is the study and use of electrical devices that operate by controlling the flow of electrons or other electrically charged particles in devices such as thermionic valves and semiconductors. The pure study of such devices is considered as a bra |
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