Logic family

Incomputer engineering,alogic familyis one of two related concepts:

• A logic family of monolithic digitalintegrated circuitdevices is a group of electroniclogic gatesconstructed using one of several different designs, usually with compatiblelogic levelsand power supply characteristics within a family. Many logic families were produced as individual components, each containing one or a few related basic logical functions, which could be used as "building-blocks" to create systems or as so-called "glue" to interconnect more complex integrated circuits.
• A logic family may also be a set of techniques used to implement logic withinVLSIintegrated circuitssuch ascentral processors,memories, or other complex functions. Some such logic families usestatic techniquesto minimize design complexity. Other such logic families, such asdomino logic,useclocked dynamic techniquesto minimize size,power consumptionand delay.

Before the widespread use of integrated circuits, various solid-state and vacuum-tube logic systems were used but these were never as standardized and interoperable as the integrated-circuit devices. The most common logic family in modernsemiconductor devicesismetal–oxide–semiconductor(MOS) logic, due to low power consumption,small transistor sizes,and hightransistor density.

Technologies

The list of packaged building-block logic families can be divided into categories, listed here in roughly chronological order of introduction, along with their usual abbreviations:

• Resistor–transistor logic(RTL)
  • Direct-coupled transistor logic(DCTL)
  • Direct-coupled unipolar transistor logic (DCUTL)
  • Resistor–capacitor–transistor logic (RCTL)
• Emitter-coupled logic(ECL)
  • Positive emitter-coupled logic(PECL)
  • Low-voltage PECL(LVPECL)
  • Complementary transistor micrologic (CTuL)
• Diode–transistor logic(DTL)
  • Complemented transistor diode logic (CTDL)
  • High-threshold logic(HTL)
• Transistor–transistor logic(TTL)
• Metal–oxide–semiconductor (MOS) logic
  • P-type MOS (PMOS) logic
  • N-type MOS (NMOS) logic
    • Depletion-load NMOS logic
    • High-density NMOS (HMOS)
  • Complementary MOS (CMOS) logic
  • Bipolar MOS (BiMOS) logic
    • Bipolar CMOS (BiCMOS)
• Integrated injection logic(I²L)
• Gunning transceiver logic(GTL)

The families (RTL, DTL, and ECL) were derived from the logic circuits used in early computers, originally implemented usingdiscrete components.One example is thePhilipsNORBITfamily of logic building blocks.

The PMOS and I²L logic families were used for relatively short periods, mostly in special purpose customlarge-scale integrationcircuits devices and are generally considered obsolete. For example, early digital clocks or electronic calculators may have used one or more PMOS devices to provide most of the logic for the finished product. TheF14 CADC,Intel 4004,Intel 4040,andIntel 8008microprocessorsand their support chips were PMOS.

Of these families, only ECL, TTL, NMOS, CMOS, and BiCMOS are currently still in widespread use. ECL is used for very high-speed applications because of its price and power demands, whileNMOS logicis mainly used inVLSIcircuits applications such as CPUs and memory chips which fall outside of the scope of this article. Present-day "building block" logic gate ICs are based on the ECL, TTL, CMOS, and BiCMOS families.

Resistor–transistor logic (RTL)

Main article:Resistor–transistor logic

Class ofdigital circuitsbuilt usingresistorsas the input network andbipolar junction transistors(BJTs) as switching devices.

TheAtanasoff–Berry Computerused resistor-coupledvacuum tubelogic circuits similar to RTL. Several earlytransistorizedcomputers (e.g.,IBM 1620,1959) used RTL, where it was implemented using discrete components.

A family of simple resistor–transistor logic integrated circuits was developed atFairchild Semiconductorfor theApollo Guidance Computerin 1962.Texas Instrumentssoon introduced its own family of RTL. A variant with integrated capacitors, RCTL, had increased speed, but lower immunity to noise than RTL. This was made by Texas Instruments as their "51XX" series.

Diode–transistor logic (DTL)

Main article:Diode–transistor logic

Class of digital circuits in which the logic gating function (e.g., AND) is performed by a diode network and the amplifying function is performed by a transistor.

Diode logicwas used with vacuum tubes in the earliest electronic computers in the 1940s includingENIAC.Diode–transistor logic (DTL) was used in theIBM 608which was the first all-transistorized computer. Early transistorized computers were implemented using discrete transistors, resistors, diodes and capacitors.

The first diode–transistor logic family of integrated circuits was introduced bySigneticsin 1962. DTL was also made by Fairchild andWestinghouse.A family of diode logic and diode–transistor logic integrated circuits was developed byTexas Instrumentsfor theD-37CMinuteman II Guidance Computerin 1962, but these devices were not available to the public.

A variant of DTL called "high-threshold logic" incorporatedZener diodesto create a large offset between logic 1 and logic 0 voltage levels. These devices usually ran off a 15 volt power supply and were found in industrial control, where the high differential was intended to minimize the effect of noise.

PMOS and NMOS logic

Main articles:PMOS logicandNMOS logic

Further information:MOSFETandDepletion-load NMOS logic

P-type MOS (PMOS) logicusesp-channelMOSFETsto implementlogic gatesand otherdigital circuits.N-type MOS (NMOS) logicusesn-channelMOSFETs to implement logic gates and other digital circuits.

For devices of equal current driving capability, n-channel MOSFETs can be made smaller than p-channel MOSFETs, due to p-channel charge carriers (holes) having lowermobilitythan do n-channel charge carriers (electrons), and producing only one type of MOSFET on a silicon substrate is cheaper and technically simpler. These were the driving principles in the design ofNMOS logicwhich uses n-channel MOSFETs exclusively. However, neglectingleakage current,unlike CMOS logic, NMOS logic consumes power even when no switching is taking place.

Mohamed M. AtallaandDawon Kahng,after they invented the MOSFET,fabricatedboth pMOS and nMOS devices with a20 μm processin 1960.Their original MOSFET devices had a gate length of 20μmand agate oxidethickness of100 nm.However, the nMOS devices were impractical, and only the pMOS type were practical working devices.A more practical NMOS process was developed several years later. NMOS was initially faster thanCMOS,thus NMOS was more widely used for computers in the 1970s.With advances in technology, CMOS logic displaced NMOS logic in the mid-1980s to become the preferred process for digital chips.

Emitter-coupled logic (ECL)

Main article:Emitter-coupled logic

ECL uses an overdrivenbipolar junction transistor(BJT) differential amplifier with single-ended input and limited emitter current.

The ECL family, ECL is also known as current-mode logic (CML), was invented by IBM ascurrent steering logicfor use in thetransistorizedIBM 7030 Stretchcomputer, where it was implemented using discrete components.

The first ECL logic family to be available in integrated circuits was introduced byMotorolaasMECLin 1962.

Transistor–transistor logic (TTL)

Main article:Transistor–transistor logic

In TTL logic,bipolar junction transistorsperform the logic and amplifying functions.

The first transistor–transistor logic family of integrated circuits was introduced bySylvaniaasSylvania Universal High–Level Logic(SUHL) in 1963. Texas Instruments introduced the7400 seriesTTL family in 1964. Transistor–transistor logic usesbipolar transistorsto form its integrated circuits.TTL has changed significantly over the years, with newer versions replacing the older types.

Since the transistors of a standard TTL gate are saturated switches, minority carrier storage time in each junction limits the switching speed of the device. Variations on the basic TTL design are intended to reduce these effects and improve speed, power consumption, or both.

The German physicistWalter H. Schottkyformulated a theory predicting theSchottky effect,which led to theSchottky diodeand laterSchottky transistors.For the same power dissipation, Schottky transistors have a faster switching speed than conventional transistors because the Schottky diode prevents the transistor from saturating and storing charge; seeBaker clamp.Gates built with Schottky transistors use more power than normal TTL and switch faster.WithLow-power Schottky(LS), internal resistance values were increased to reduce power consumption and increase switching speed over the original version. The introduction ofAdvanced Low-power Schottky(ALS) further increased speed and reduced power consumption. A faster logic family calledFAST(Fairchild Advanced Schottky TTL) (Schottky) (F) was also introduced that was faster than normal Schottky TTL.

Complementary MOS (CMOS) logic

Main article:CMOS

CMOS logic gates use complementary arrangements of enhancement-mode N-channel and P-channelfield effect transistor.Since the initial devices used oxide-isolated metal gates, they were calledCMOS(complementary metal–oxide–semiconductor logic). In contrast to TTL, CMOS uses almost no power in the static state (that is, when inputs are not changing). A CMOS gate draws no current other than leakage when in a steady 1 or 0 state. When the gate switches states, current is drawn from the power supply to charge the capacitance at the output of the gate. This means that the current draw of CMOS devices increases with switching rate (controlled by clock speed, typically).

The first CMOS family of logic integrated circuits was introduced byRCAasCD4000 COS/MOS,the4000 series,in 1968. Initially CMOS logic was slower than LS-TTL. However, because the logic thresholds of CMOS were proportional to the power supply voltage, CMOS devices were well-adapted to battery-operated systems with simple power supplies. CMOS gates can also tolerate much wider voltage ranges than TTL gates because the logic thresholds are (approximately) proportional to power supply voltage, and not the fixed levels required by bipolar circuits.

The required silicon area for implementing such digital CMOS functions has rapidly shrunk.VLSI technologyincorporating millions of basic logic operations onto one chip, almost exclusively uses CMOS. The extremely small capacitance of the on-chip wiring caused an increase in performance by several orders of magnitude. On-chip clock rates as high as 4 GHz have become common, approximately 1000 times faster than the technology by 1970.

Lowering the power supply voltage

Main article:LVCMOS

CMOS chips often work with a broader range of power supply voltages than other logic families. Early TTL ICs required apower supplyvoltageof 5V, but early CMOS could use 3 to 15V.Lowering the supply voltage reduces the charge stored on any capacitances and consequently reduces the energy required for a logic transition. Reduced energy implies less heat dissipation. The energy stored in a capacitanceCand changingVvolts is ½CV².By lowering the power supply from 5V to 3.3V, switching power was reduced by almost 60 percent (power dissipationis proportional to the square of the supply voltage). Many motherboards have avoltage regulator moduleto provide the even lower power supply voltages required by many CPUs.

HC logic

Because of the incompatibility of the CD4000 series of chips with the previous TTL family, a new standard emerged which combined the best of the TTL family with the advantages of the CD4000 family. It was known as the 74HC (which used anywhere from 3.3V to 5V power supplies (and used logic levels relative to the power supply)), and with devices that used 5V power supplies and TTLlogic levels.

The CMOS–TTL logic level problem

Interconnecting any two logic families often required special techniques such as additionalpull-up resistors,or purpose-built interface circuits, since the logic families may use differentvoltage levelsto represent 1 and 0 states, and may have other interface requirements only met within the logic family.

TTL logic levels are different from those of CMOS – generally a TTL output does not rise high enough to be reliably recognized as a logic 1 by a CMOS input. This problem was solved by the invention of the 74HCT family of devices that uses CMOS technology but TTL input logic levels. These devices only work with a 5V power supply. They form a replacement for TTL, although HCT is slower than original TTL (HC logic has about the same speed as original TTL).

Other CMOS families

Other CMOS circuit families withinintegrated circuitsincludecascode voltage switch logic(CVSL) andpass transistor logic(PTL) of various sorts. These are generally used "on-chip" and are not delivered as building-block medium-scale or small-scale integrated circuits.

Bipolar CMOS (BiCMOS) logic

Main article:BiCMOS

One major improvement was to combine CMOS inputs and TTL drivers to form of a new type of logic devices calledBiCMOS logic,of which the LVT and ALVT logic families are the most important. The BiCMOS family has many members, includingABT logic,ALB logic,ALVT logic,BCT logicandLVT logic.

Improved versions

With HC and HCT logic and LS-TTL logic competing in the market it became clear that further improvements were needed to create theideallogic device that combined high speed, with low power dissipation and compatibility with older logic families. A whole range of newer families has emerged that use CMOS technology. A short list of the most important family designators of these newer devices includes:

• LV logic(lower supply voltage)
• LVT logic(lower supply voltage while retaining TTL logic levels)
• ALVT logic(an 'advanced' version of LVT logic)

There are many others includingAC/ACT logic,AHC/AHCT logic,ALVC logic,AUC logic,AVC logic,CBT logic,CBTLV logic,FCT logicandLVC logic(LVCMOS).

Integrated injection logic (IIL)

Main article:Integrated injection logic

The integrated injection logic (IIL or I²L) usesbipolar transistorsin a current-steering arrangement to implement logic functions.It was used in some integrated circuits, but it is now considered obsolete.

Monolithic integrated circuit logic families compared

The following logic families would either have been used to build up systems from functional blocks such as flip-flops, counters, and gates, or else would be used as "glue" logic to interconnect very-large scale integration devices such as memory and processors. Not shown are some early obscure logic families from the early 1960s such as DCTL (direct-coupled transistor logic), which did not become widely available.

Propagation delayis the time taken for a two-input NAND gate to produce a result after a change of state at its inputs.Toggle speedrepresents the fastest speed at which a J-K flip flop could operate.Power per gateis for an individual 2-input NAND gate; usually there would be more than one gate per IC package. Values are very typical and would vary slightly depending on application conditions, manufacturer, temperature, and particular type of logic circuit.Introduction yearis when at least some of the devices of the family were available in volume for civilian uses. Some military applications pre-dated civilian use.

Family	Description	Propagation delay (ns)	Toggle speed (MHz)	Power per gate @1 MHz (mW)	Typical supply voltage V (range)	Introduction year	Remarks
RTL	Resistor–transistor logic	500	4	10	3.3	1963	the first CPU built from integrated circuits (theApollo Guidance Computer) used RTL.
DTL	Diode–transistor logic	25		10	5	1962	Introduced by Signetics, Fairchild 930 line became industry standard in 1964
PMOS	MEM 1000	300	1	9	-27 and -13	1967	Introduced by General Instrument
CMOS	AC/ACT	3	125	0.5	3.3 or 5 (2-6 or 4.5-5.5)	1985	ACT has TTL compatible levels
CMOS	HC/HCT	9	50	0.5	5 (2-6 or 4.5-5.5)	1982	HCT has TTL compatible levels
CMOS	4000B/74C	30	5	1.2	10V (3-18)	1970	Approximately half speed and power at 5 volts
TTL	Original series	10	25	10	5 (4.75-5.25)	1964	Several manufacturers
TTL	L	33	3	1	5 (4.75-5.25)	1964	Low power
TTL	H	6	43	22	5 (4.75-5.25)	1964	High speed
TTL	S	3	100	19	5 (4.75-5.25)	1969	Schottky high speed
TTL	LS	10	40	2	5 (4.75-5.25)	1976	Low power Schottky high speed
TTL	ALS	4	50	1.3	5 (4.5-5.5)	1976	Advanced Low power Schottky
TTL	F	3.5	100	5.4	5 (4.75-5.25)	1979	Fast
TTL	AS	2	105	8	5 (4.5-5.5)	1980	Advanced Schottky
TTL	G	1.5	1125 (1.125 GHz)		1.65 - 3.6	2004	First GHz 7400 series logic
ECL	ECL III	1	500	60	-5.2(-5.19 - -5.21)	1968	Improved ECL
ECL	MECL I	8		31	-5.2	1962	first integrated logic circuit commercially produced
ECL	ECL 10K	2	125	25	-5.2(-5.19 - -5.21)	1971	Motorola
ECL	ECL 100K	0.75	350	40	-4.5(-4.2 - -5.2)	1981
ECL	ECL 100KH	1	250	25	-5.2(-4.9 - -5.5)	1981

On-chip design styles

Several techniques and design styles are primarily used in designing large single-chip application-specific integrated circuits (ASIC) and CPUs, rather than generic logic families intended for use in multi-chip applications.

These design styles can typically be divided into two main categories, static techniquesand clocked dynamic techniques. (Seestatic versus dynamic logicfor some discussion on the advantages and disadvantages of each category).

Static logic

• Pulsed static CMOS
• Differential cascode voltage switch (DCVS)
• Cascode non-threshold logic (CNTL)
• Pass-gate/transmission-gate logic:pass transistor logic(PTL)
• Complementary pass-gate logic (CPL)
• Push–pull logic
• Output prediction logic (OPL)
• Cascode voltage switch logic(CVSL)

Dynamic logic

• four-phase logic
• domino logic
• Footless domino
• NORA/zipper logic
• Multiple-output domino
• Compound domino
• Dual-rail domino
• Self-resetting domino
• Sample-set differential logic
• Limited-switch dynamic logic

See also

• 4000 series(commercial CMOS logic)
• 7400 series(commercial transistor-transistor logic)
• Logic gate
• Pin-compatibility
• Open collector

Further reading

• Westman, H.P. (ed.).Reference Data for Radio Engineers(6th ed.). Howard W. Sams & Co.ISBN0-672-21218-8.OCLC919405869.4th ed. 1964
• Savard, John J. G. (2018) [2005]."What Computers Are Made From".quadibloc.Archivedfrom the original on 2018-07-02.Retrieved2018-07-16.
• Texas Instrument."Logic Guide"(PDF).