 # Learning Resources

## Digital electronics

igital electronics, or digital (electronic) circuits, represent signals by discrete bands of analog levels, rather than by a continuous range. All levels within a band represent the same signal state. Relatively small changes to the analog signal levels due to manufacturing tolerance, signal attenuation or parasitic noise do not leave the discrete envelope, and as a result are ignored by signal state sensing circuitry.

In most cases the number of these states is two, and they are represented by two voltage bands: one near a reference value (typically termed as "ground" or zero volts) and a value near the supply voltage, corresponding to the "false" ("0") and "true" ("1") values of the Boolean domain respectively.

Digital techniques are useful because it is easier to get an electronic device to switch into one of a number of known states than to accurately reproduce a continuous range of values.

Digital electronic circuits are usually made from large assemblies of logic gates, simple electronic representations of Boolean logic functions

Gates are the fundamental building blocks of digital logic circuitry. These devices function by “opening” or “closing” to admit or reject the passage of a logical signal. From only a handful of basic gate types (AND, OR, XOR, and NOT), a vast array of gating functions can be created.

In digital electronics, the on state is often represented by a 1 and the off state by a 0. The relationship between the input signals and the output signals is often summarized in a truth table, which is a tabulation of all possible inputs and the resulting outputs.

Gates
Logic gates (or simply gates) are the fundamental building blocks of digital circuitry. As their name implies, they function by “opening” or “closing” to admit or reject the flow of digital information. Gates implement electronically simple logical opera­tions on boolean (Bool’s algebra) variables, i.e. variables that can have only one of two states (0/1, low/high, false/true). From an electrical point of view and for the TTL (transistor-transistor-logic) family of digital electronics, any voltage in the range 0-0,7 V and in the range 2,5-5 V, represent logic states 0 and 1, respectively. In the following figure the accepted electronic symbols for different gates are shown, along with their corresponding “truth tables” and their symbolic logical expressions. All variables (X, A, B, …) are booleans.

Multiplexers
Generally, multiplexers are circuits behaving like a controlled rotary switch, i.e. any one of a number of inputs may be selected as output. In digital electronics, a multiplexer is a combination of logic gates resulting into circuits with two or more inputs (data inputs) and one output. The selection of the channel to be read into the output is controlled by supplying a specific digital word to a different set of inputs (select inputs).