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23-466: Fan out = C load / C in , where As a delay metric, one FO4 is the delay of an inverter , driven by an inverter 4x smaller than itself, and driving an inverter 4x larger than itself. Both conditions are necessary since input signal rise/fall time affects the delay as well as output loading. FO4 is generally used as a delay metric because such a load is generally seen in case of tapered buffers driving large loads, and approximately in any logic gate of

46-451: A CMOS configuration. This configuration greatly reduces power consumption since one of the transistors is always off in both logic states. Processing speed can also be improved due to the relatively low resistance compared to the NMOS-only or PMOS-only type devices. Inverters can also be constructed with bipolar junction transistors (BJT) in either a resistor–transistor logic (RTL) or

69-545: A transistor–transistor logic (TTL) configuration. Digital electronics circuits operate at fixed voltage levels corresponding to a logical 0 or 1 (see binary ). An inverter circuit serves as the basic logic gate to swap between those two voltage levels. Implementation determines the actual voltage, but common levels include (0, +5V) for TTL circuits. The inverter is a basic building block in digital electronics. Multiplexers, decoders, state machines, and other sophisticated digital devices may use inverters. The hex inverter

92-402: A logic path sized for minimum delay. Also, for most technologies the optimum fanout for such buffers generally varies from 2.7 to 5.3. A fan out of 4 is the answer to the canonical problem stated as follows: Given a fixed size inverter, small in comparison to a fixed large load, minimize the delay in driving the large load. After some math, it can be shown that the minimum delay is achieved when

115-460: A resistor between the output and input. NOR gate The NOR gate is a digital logic gate that implements logical NOR - it behaves according to the truth table to the right. A HIGH output (1) results if both the inputs to the gate are LOW (0); if one or both input is HIGH (1), a LOW output (0) results. NOR is the result of the negation of the OR operator. It can also in some senses be seen as

138-425: A single PMOS transistor coupled with a resistor . Since this "resistive-drain" approach uses only a single type of transistor, it can be fabricated at a low cost. However, because current flows through the resistor in one of the two states, the resistive-drain configuration is disadvantaged for power consumption and processing speed. Alternatively, inverters can be constructed using two complementary transistors in

161-431: A small circle or "bubble". Input and output lines are attached to the symbol; the bubble is typically attached to the output line. To symbolize active-low input , sometimes the bubble is instead placed on the input line. Sometimes only the circle portion of the symbol is used, and it is attached to the input or output of another gate; the symbols for NAND and NOR are formed in this way. A bar or overline ( ‾ ) above

184-419: A variable can denote negation (or inversion or complement) performed by a NOT gate. A slash (/) before the variable is also used. An inverter circuit outputs a voltage representing the opposite logic-level to its input. Its main function is to invert the input signal applied. If the applied input is low then the output becomes high and vice versa. Inverters can be constructed using a single NMOS transistor or

207-497: Is a logic gate which implements logical negation . It outputs a bit opposite of the bit that is put into it. The bits are typically implemented as two differing voltage levels. The NOT gate outputs a zero when given a one, and a one when given a zero. Hence, it inverts its inputs. Colloquially, this inversion of bits is called "flipping" bits. As with all binary logic gates, other pairs of symbols — such as true and false, or high and low — may be used in lieu of one and zero. It

230-411: Is an integrated circuit that contains six ( hexa- ) inverters. For example, the 7404 TTL chip which has 14 pins and the 4049 CMOS chip which has 16 pins, 2 of which are used for power/referencing, and 12 of which are used by the inputs and outputs of the six inverters (the 4049 has 2 pins with no connection). f ( a ) = 1 − a {\displaystyle f(a)=1-a}

253-414: Is equivalent to the logical negation operator (¬) in mathematical logic . Because it has only one input, it is a unary operation and has the simplest type of truth table . It is also called the complement gate because it produces the ones' complement of a binary number, swapping 0s and 1s. The NOT gate is one of three basic logic gates from which any Boolean circuit may be built up. Together with

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276-481: Is more fair to normalize each adder's latency to the delay of one FO4 inverter. The FO4 time for a technology is five times its RC time constant τ; therefore 5·τ = FO4. Some examples of high-frequency CPUs with long pipeline and low stage delay: IBM Power6 has design with cycle delay of 13 FO4; clock period of Intel's Pentium 4 at 3.4 GHz is estimated as 16.3 FO4. Inverter (logic gate) In digital logic, an inverter or NOT gate

299-494: Is pulled high through the pull-up resistor . The diagram below shows a 2-input NOR gate using CMOS technology. The diodes and resistors on the inputs are to protect the CMOS components from damage due to electrostatic discharge (ESD) and play no part in the logical function of the circuit. In CMOS, NOR gates are less efficient than NAND gates . This is due to the faster charge mobility in n-MOSFETs compared to p-MOSFETs, so that

322-448: Is the analytical representation of NOT gate: If no specific NOT gates are available, one can be made from the universal NAND or NOR gates, or an XOR gate by setting one input to high. Digital inverter quality is often measured using the voltage transfer curve (VTC), which is a plot of output vs. input voltage. From such a graph, device parameters including noise tolerance, gain, and operating logic levels can be obtained. Ideally,

345-618: Is the more complicated operation; it may use a NOR followed by a NOT. A significant exception is some forms of the domino logic family. There are three symbols for NOR gates: the American (ANSI or 'military') symbol and the IEC ('European' or 'rectangular') symbol, as well as the deprecated DIN symbol. For more information see Logic Gate Symbols . The ANSI symbol for the NOR gate is a standard OR gate with an inversion bubble connected. The bubble indicates that

368-495: The AND gate and the OR gate , any function in binary mathematics may be implemented. All other logic gates may be made from these three. The terms "programmable inverter" or "controlled inverter" do not refer to this gate; instead, these terms refer to the XOR gate because it can conditionally function like a NOT gate. The traditional symbol for an inverter circuit is a triangle touching

391-527: The VTC appears as an inverted step function – this would indicate precise switching between on and off – but in real devices, a gradual transition region exists. The VTC indicates that for low input voltage, the circuit outputs high voltage; for high input, the output tapers off towards the low level. The slope of this transition region is a measure of quality – steep (close to vertical) slopes yield precise switching. The tolerance to noise can be measured by comparing

414-426: The fan out of 4 as a metric. For example, given two 64-bit adders, one implemented in a 0.5 μm technology and the other in 90 nm technology, it would be unfair to say the 90 nm adder is better from a circuits and architecture standpoint just because it has less latency. The 90 nm adder might be faster only due to its inherently faster devices. To compare the adder architecture and circuit design, it

437-609: The function of the or gate has been inverted. NOR Gates are basic logic gates, and as such they are recognised in TTL and CMOS ICs . The standard, 4000 series , CMOS IC is the 4001, which includes four independent, two-input, NOR gates. The pinout diagram is as follows: These devices are available from most semiconductor manufacturers such as Fairchild Semiconductor , Philips or Texas Instruments . These are usually available in both through-hole DIP and SOIC format. Datasheets are readily available in most datasheet databases . In

460-462: The inverse of an AND gate . NOR is a functionally complete operation—NOR gates can be combined to generate any other logical function. It shares this property with the NAND gate . By contrast, the OR operator is monotonic as it can only change LOW to HIGH but not vice versa. In most, but not all, circuit implementations, the negation comes for free—including CMOS and TTL . In such logic families, OR

483-596: The load is driven by a chain of N inverters, each successive inverter ~4x larger than the previous; N ~ log 4 (C load /C in ) . In the absence of parasitic capacitances (drain diffusion capacitance and wire capacitance), the result is "a fan out of e" (now N ~ ln(C load /C in ). If the load itself is not large, then using a fan out of 4 scaling in successive logic stages does not make sense. In these cases, minimum sized transistors may be faster. Because scaled technologies are inherently faster (in absolute terms), circuit performance can be more fairly compared using

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506-416: The minimum input to the maximum output for each region of operation (on / off). Since the transition region is steep and approximately linear, a properly-biased CMOS inverter digital logic gate may be used as a high-gain analog linear amplifier or even combined to form an opamp . Maximum gain is achieved when the input and output operating points are the same voltage, which can be biased by connecting

529-406: The popular CMOS and TTL logic families , NOR gates with up to 8 inputs are available: In the older RTL and ECL families, NOR gates were efficient and most commonly used. The diagrams above show the construction of a 2-input NOR gate using NMOS logic circuitry. If either of the inputs is high, the corresponding N-channel MOSFET is turned on and the output is pulled low; otherwise the output

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