Logic family

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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.

Contents

[edit] 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:

The four families (DL, RTL, DTL, and ECL) were originally implemented using discrete components, in circuits derived from the vacuum tube based logic of early computers. Diode logic was never implemented in integrated circuits, as DTL was far more practical.

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.

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.

[edit] ECL

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

[edit] RTL

A family of simple resistor-transistor logic integrated circuits was developed for the Minuteman II Guidance Computer and Apollo Guidance Computer in 1962, but these devices were not available to the public.

Both Texas Instruments and Fairchild Semiconductor soon introduced their own families 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.

[edit] DTL

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

[edit] 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.

[edit] 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.

[edit] 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.

[edit] 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.

[edit] 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.

[edit] 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.

[edit] 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.

[edit] 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.

[edit] 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.

[edit] Other families

Other circuit families include:

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

[edit] 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.

[edit] External link

[edit] References

  • Paul Horowitz and Winfield Hill, The Art of Electronics 2nd Ed. Cambridge University Press, Cambridge, 1989 ISBN 0-521-37095-7
  • 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
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