logic gate

A digital logic gate can have more than one input, for example, inputs A, B, C, D etc., but generally only have one digital output, (Q). Individual logic gates can be connected or cascaded together to form a logic gate function with any desired number of inputs, or to form combinational and sequential type circuits, or to produce differnt logic gate functions from standard gates. Standard commercially available digital logic gates are available in two basic families or forms, TTL which stands for Transistor-Transistor Logic such as the 7400 series, and CMOS which stands for Complementary Metal-Oxide-Silicon which is the 4000 series of chips. This notation of TTL or CMOS refers to the logic technology used to manufacture the integrated circuit, (IC) or a “chip” as it is more commonly called. Generally speaking, TTL logic IC’s use NPN and PNP type Bipolar Junction Transistors while CMOS logic IC’s use complementary MOSFET or JFET type Field Effect Transistors for both their input and output circuitry. As well as TTL and CMOS technology, simple digital logic gates can also be made by connecting together diodes, transistors and resistors to produce RTL, Resistor-Transistor logic gates, DTL, Diode-Transistor logic gates or ECL, Emitter-Coupled logic gates but these are less common now compared to the popular CMOS family. Integrated Circuits or IC’s as they are more commonly called, can be grouped together into logic families according to the number of individual transistors or “gates” that they may contain within their design. For example, a simple AND gate may contain only a few individual transistors to operate. Whereas a more complex microprocessor chip can contain billions of individual transistor gates on one single wafer. Integrated circuits are categorised according to the number of logic gates or the complexity of the circuits within a single chip with the general classification for the number of individual gates given as: ■ Small Scale Integration or (SSI) – Contain up to 10 transistors or a few gates within a single package such as AND, OR, NOT gates. ■ Medium Scale Integration or (MSI) – between 10 and 100 transistors or tens of gates within a single package and perform digital operations such as adders, decoders, counters, flip-flops and multiplexers. ■ Large Scale Integration or (LSI) – between 100 and 1,000 transistors or hundreds of gates and perform specific digital operations such as I/O chips, memory, arithmetic and logic units. ■ Very-Large Scale Integration or (VLSI) – between 1,000 and 10,000 transistors or thousands of gates and perform computational operations such as processors, large memory arrays and programmable logic devices. ■ Super-Large Scale Integration or (SLSI) – between 10,000 and 100,000 transistors within a single package and perform computational operations such as microprocessor chips, micro-controllers, basic PICs and calculators. ■ Ultra-Large Scale Integration or (ULSI) – more than 1 million transistors – the big boys that are used in computers CPUs, GPUs, video processors, micro-controllers, FPGAs and complex PICs. While the “ultra large scale” ULSI classification is less well used, another level of integration which represents the complexity of the Integrated Circuit is known as the System-on-Chip or ( SOC ) for short. Here the individual components such as the microprocessor, memory, peripherals, I/O logic etc, are all produced on a single piece of silicon and which represents a whole electronic system within one single chip, literally putting the word “integrated” into integrated circuit. These complete integrated chips which can contain up to 100 million individual silicon-CMOS transistor gates within one single package are generally used in mobile phones, digital cameras, micro-controllers, PIC’s and robotic type applications. In 1965, Gordon Moore co-founder of the Intel corporation predicted that “The number of transistors and resistors on a single chip will double every 18