Logic family: Difference between revisions

In computer engineering, a logic family may refer to one of two related concepts. A logic family of monolithic digital integrated circuit devices is a group of electronic logic gates constructed using one of several different designs, usually with compatible logic levels and 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 refer to a set of techniques used to implement logic within large scale integrated circuits such as a central processor, memory, or other complex function. Such logic families use clocked dynamic techniques to minimize power consumption and 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 beginning

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

• Diode logic (DL)
• Direct-coupled transistor logic (DCTL)
• Complementary transistor logic (CTL)
• Resistor-transistor logic (RTL)
• Resistor-capacitor transistor logic (RCTL)
• Diode-transistor logic (DTL)
• Emitter coupled logic (ECL) also known as Current-mode logic (CML)
• Transistor-transistor logic (TTL) and variants
• P-type Metal Oxide Semiconductor logic (PMOS)
• N-type Metal Oxide Semiconductor logic (NMOS)
• Complementary Metal-Oxide Semiconductor logic (CMOS)
• Bipolar Complementary Metal-Oxide Semiconductor logic (BiCMOS)
• Integrated Injection Logic (I²L)

The families (DL, RTL, DTL, and ECL) were derived from the logic circuits used in early computers, originally implemented using discrete components. One example is the Philips NORbits family of logic building blocks.

The PMOS and I²L logic families were used for relatively short periods, mostly in special purpose custom LSI (Large Scale Integrated Circuits) 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. The F14 CADC, Intel 4004, Intel 4040, and Intel 8008 microprocessors and their support chips were PMOS.

Of these families, only five (ECL, TTL, CMOS, NMOS, and BiCMOS) are currently still in widespread use. ECL is used for very high speed applications because of its price and power demands, while NMOS logic is mainly used in VLSI (Very Large Scale Integrated Circuits) 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.

ECL

The ECL logic family was invented by IBM as current steering logic for use in the transistorized IBM 7030 Stretch computer, where it was implemented using discrete components.

The first logic family to be available in integrated circuits was emitter coupled logic, introduced by Motorola as MECL in 1962..

RTL

The Atanasoff–Berry Computer used vacuum tube logic circuits similar to RTL. Several early transistorized computers (e.g., IBM 1620) used RTL, where it was implemented using discrete components.

A family of simple resistor-transistor logic integrated circuits was developed at Fairchild Semiconductor for the Apollo Guidance Computer in 1962.

Texas Instruments soon 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.

DTL

Diode logic goes back as far as ENIAC and was used in many early vacuum tube computers. Several early transistorized computers (e.g., IBM 1401) used DTL, where it was implemented using discrete components.

The first diode-transistor logic family of integrated circuits was introduced by Signetics in 1962. DTL was also made by Fairchild and Westinghouse.

A family of diode logic and diode-transistor logic integrated circuits was developed by Texas Instruments for the D-37C Minuteman II Guidance Computer in 1962, but these devices were not available to the public.

HTL

A variant of DTL called "high threshold logic" incorporated Zener diodes to 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.

TTL

Main article: Transistor-transistor logic

The first transistor-transistor logic family of integrated circuits was introduced by Sylvania as Sylvania Universal High–Level Logic (SUHL) in 1963. Texas Instruments introduced 5400 Series TTL family in 1964.

Transistor-Transistor Logic uses bipolar transistors to form its integrated circuits. TTL has changed significantly over the years, with newer versions replacing the older types.

LS TTL

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 physicist Walter H. Schottky formulated a theory predicting the Schottky effect, which led to the Schottky diode and later Schottky transistors. Schottky transistors have a much higher switching speed than conventional transistors because the Schottky junction does not promote charge storage, leading to faster switching gates. Gates built with Schottky transistors use more power than normal TTL and switch faster. With low power Schottky circuits, internal resistance values were increased to reduce power consumption and increase switching speed over the original version. The introduction of Advanced Low Power(ALS) further increased speed and reduced power consumption. A faster logic family called 'Fast TTL' (F) was also introduced that was faster than normal Schottky TTL.

CMOS

CMOS gates use a complementary pair of FETs (a N-MOS and a P-MOS FET). Since the initial devices used metal layers for gate contacts, they were called CMOS (Complementary Metal Oxide Semiconductor logic).

In contrast to TTL logic, CMOS uses almost no power in the static state (i.e. 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 stray capacitance at the output of the gate. This means that current draw of CMOS devices increases with clock speed.

The first CMOS logic family of integrated circuits was introduced by RCA as CD4000 COS/MOS, the 4000 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.

With this technology the required silicon area for implementing such digital CMOS functions has rapidly shrunk. Nowadays, the modern VLSI technology, allowing for incorporating many millions of basic logic operations onto one single silicon chip, almost exclusively exploits this CMOS technology. The extremely small capacitance of the on chip wiring, as well as its length compared to a PCB, combined with the absence of the delay created by the required I/O-cells of all separate gate chips, 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

One very important feature of CMOS chips is that they work with a broader range of power supply voltages. While TTL ICs all require a power supply voltage of 5V (+/- 0.5V), CMOS works with a wider range of power supply voltage -- usually anywhere from 3 to 15V. Lowering the supply voltage reduces the current required to charge stray capacitance, and so reduces the current drawn by complex microprocessors. This in turn reduces the heat dissipation of the processor. By lowering the power supply from 5V to 3.3V, switching power was reduced by almost 60 percent (power dissipation is proportional to the square of the supply voltage). Newer CPUs have lowered their power supply voltages further.

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 (High performance silicon gate) family of devices and used the pinout of the 74LS family with an improved version of CMOS technology inside the chip. And it could be used together with other logic devices which used 3.3V power supplies (and thus 3.3V logic levels), and in designs that used 5V power supplies and devices that used TTL logic levels.

The logic level problem

There was however a problem when combining CMOS and TTL logic. 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 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 logic, although HCT is slower than original TTL (HC logic has about the same speed as original TTL).

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

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 the 'ideal' logic 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)

But there are many others including AC/ACT logic, AHC/AHCT logic, ALVC logic, AUC logic, AVC logic, CBT logic, CBTLV logic, FCT logic, LVC logic and LVC logic.

BiCMOS

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

Other families

Other circuit families include:

• Complementary Pass-transistor Logic (CPL)
• Cascode Voltage Switch Logic (CVSL)

These are generally used "on-chip" and are not delivered as building-block medium-scale or small-scale integrated circuits.

Conclusion

At the moment the most common logic families still are the LS-TTL and the HC and HCT families, although BCT logic also is starting to become popular. Low voltage logic families are becoming more important, with AHC logic gaining popularity. Because not every manufacturer supports every logic family, several families of logic circuits will continue to be used.

Monolithic integrated circuit logic families table

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 1960's such as direct-coupled transistor logic, which did not become widely available.

"Propagation delay" is the time taken for a two-input NAND gate to produce a result after a change of state at its inputs. "Toggle speed" represents the fastest speed at which a J-K flip flop could operate. "Power per gate" is for an individual gate such as a 2-input NAND; often 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.

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	__	4	10	3.3	1963	First monolithic integrated circuit family
DTL	Diode-transistor logic	__	__	10	5	1965
CMOS	ACT/ACT	3	125	0.5	3.3 or 5 (2-6 or 4.5 to 5.5)	1985	ACT has TTL Compatible levels
CMOS	HC/HCT	9	30	0.5	5 (2-6)	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	110	19	5 (4.75-5.25)	1969	Schottky high speed
TTL	LS	10	33	2	5 (4.75-5.25)	1976	Low power Schotky high speed
TTL	ALS	4	34	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	AS	2	105	8	5 (4.5-5.5)	1980	Advanced Schottky
ECL	ECL III	1	500	60	-5.2(-5.19 - -5.21)	1968	Original ECL
ECL	ECL 10K	2	125	25	-5.2(-5.19 - -5.21)	1971	Motorola
ECL	ECL 100K	.75	350	40	-4.5(-4.2 - -5.2)	1981
ECL	ECL 100KH	1	250	25	-5.2(-4.9 - -5.5)	1981
ECL	ECL 100K	.75	350	40	-4.5(-4.2 - -5.2)	1981

External links

• What Computers are Made From

References

• Paul Horowitz and Winfield Hill, The Art of Electronics 2nd Ed. Cambridge University Press, Cambridge, 1989 ISBN 0-521-37095-7
• Don Lancaster, TTL Cookbook, Howard W. Sams and Co., Indianapolis, 1975, ISBN 0-672-21035-5
• The Engineering Staff, The TTL Data Book for Design Engineers, 1st Ed., Texas Instruments, Dallas Texas, 1973, no ISBN
• H. P. Westman (ed), Reference Data for Radio Engineers 5th Edition, Howard W. Sams & Co., Indianapolis, 1968, no ISBN, Library of Congress Card 43-14665