transistor-transistor logic

Transistor–transistor logic

Transistor–transistor logic (TTL) is a class of digital circuits built from bipolar junction transistors (BJT), and resistors. It is called transistor–transistor logic because both the logic gating function (e.g., AND) and the amplifying function are performed by transistors (contrast this with RTL and DTL).

TTL is notable for being a widespread integrated circuit (IC) family used in many applications such as computers, industrial controls, test equipment and instrumentation, consumer electronics, synthesizers, etc. The designation TTL is sometimes used to mean TTL-compatible logic levels, even when not associated directly with TTL integrated circuits, for example as a label on the inputs and outputs of electronic instruments.

History

TTL logic was invented in 1961 by James L. Buie of TRW, "particularly suited to the newly developing integrated circuit design technology.

The first commercial integrated-circuit TTL devices were manufactured by Sylvania in 1963, called the Sylvania Universal High-Level Logic family (SUHL). The Sylvania parts were used in the controls of the Phoenix missile. TTL became popular with electronic systems designers after Texas Instruments introduced the 5400 series with military temperature range in 1964 and the later 7400 series of ICs, specified over a lower range, in 1966.

The Texas Instruments 7400 family became an industry standard. Compatible parts were made by Motorola, AMD, Fairchild, Intel, Intersil, Signetics,Mullard, Siemens. SGS-Thomson, and National Semiconductor, and many other companies, even in the former Soviet Union. Not only did third parties make compatible TTL parts, but compatible parts were made using many other circuit technologies as well.

The term "TTL" is applied to many successive generations of bipolar logic, with gradual improvements in speed and power consumption over about two decades. The last widely available family, 74AS/ALS Advanced Schottky, was introduced in 1985. As of 2008, Texas Instruments continues to supply the more general-purpose chips in numerous obsolete technology families, albeit at increased prices. Typically, TTL logic chips integrate no more than a few hundred transistors. Functions within a single package generally range from a few logic gates to a microprocessor bit-slice. TTL also became important because its low cost made digital techniques economically practical for tasks previously done by analog methods.

The Kenbak-1, one of the first personal computers, used TTL for its CPU instead of a microprocessor chip, which was not available in 1971. The 1973 Xerox Alto and 1981 Star workstations, which introduced the graphical user interface, used TTL circuits integrated at the level of ALUs and bitslices, respectively. Most computers used TTL-compatible logic between larger chips well into the 1990s. Until the advent of programmable logic, discrete bipolar logic was used to prototype and emulate microarchitectures under development.

Theory

TTL contrasts with the preceding resistor–transistor logic (RTL) and diode–transistor logic (DTL) generations by using transistors not only to amplify the output, but also to isolate the inputs. The p-n junction of a diode has considerable capacitance, so changing the logic level of an input connected to a diode, as in DTL, requires considerable time and energy.

As shown in the top schematic at right, the fundamental concept of TTL is to isolate the inputs by using a common-base connection, and amplify the function using a common emitter connection. Note that the base of the output transistor is driven high only by the forward-biased base–collector junction of the input transistor. The second schematic adds to this a "totem-pole output". When V2 is off (output equals 1), the resistors turn V3 on and V4 off, resulting in a stronger 1 output. When V2 is on, it activates V4, driving 0 to the output. The diode forces the emitter of V3 to ~0.7 V, while R2, R4 are chosen to pull its base to a lower voltage, turning it off. By removing pull-up and pull-down transistors from the output stage, this allows the strength of the gate to be increased without proportionally affecting power consumption.

TTL is particularly well suited to integrated circuits because the inputs of a gate may all be integrated into a single base region to form a multiple-emitter transistor. Such a highly customized part might increase the cost of a circuit where each transistor is in a separate package. However, by combining several small on-chip components into one larger device, it reduces the cost of implementation on an IC.

As with all bipolar logic, a small amount of current must be drawn from a TTL input to ensure proper logic levels. The total current drawn must be within the capacities of the preceding stage, which limits the number of nodes that can be connected (the fanout).

All standardized common TTL circuits operate with a 5-volt power supply. A TTL input signal is defined as "low" when between 0 V and 0.8 V with respect to the ground terminal, and "high" when between 2.2 V and 5 V (precise logic levels vary slightly between sub-types). Standardization of the TTL logic levels was so ubiquitous that complex circuit boards often contained TTL chips made by many manufacturers, selected for availability and cost and not just compatibility. Within usefully broad limits, logic gates could be treated as ideal Boolean devices without concern for electrical limitations.

Like most integrated circuits of the period 1965–1990, TTL devices were usually packaged in through-hole, dual in-line packages with between 14 and 24 lead wires, usually made of epoxy plastic or sometimes made of ceramic. Beam-lead chips without packages were made for assembly into larger arrays as hybrid integrated circuits. Parts for military and aerospace applications were packaged in flat packs, a form of surface-mount package, with leads suitable for welding or soldering to printed circuit boards. Today, many TTL-compatible devices are available in surface-mounted packages.

Comparison with other logic families

TTL devices consume substantially more power than an equivalent CMOS device at rest, but power consumption does not increase with clock speed as rapidly as for CMOS devices. Compared to contemporary ECL circuits, TTL uses less power and has easier design rules, but is substantially slower. Designers can combine ECL and TTL devices in the same system to achieve best overall performance and economy, but level-shifting devices were required between the two logic families. TTL was less sensitive to damage from electrostatic discharge than early CMOS devices.

Due to the output structure of TTL devices, the output impedance is asymmetrical between the high and low state, making them unsuitable for driving transmission lines. This is usually solved by buffering the outputs with special line driver devices where signals need to be sent through cables. ECL, by virtue of its symmetric low-impedance output structure, does not have this drawback.

The TTL "totem-pole" output structure often has a momentary overlap between the upper and lower transistors, resulting in a substantial pulse of current drawn from the supply. These pulses can couple in unexpected ways between multiple integrated circuit packages, resulting in reduced noise margin and lower performance. TTL systems usually have a decoupling capacitor for every one or two IC packages, so that a current pulse from one chip does not momentarily reduce the supply voltage to the others.

Several manufacturers now supply CMOS logic equivalents with TTL-compatible input and output levels, usually bearing part numbers similar to the equivalent TTL component and with the same pin-out diagrams. For example, the 74HCT00 series provides many drop-in replacements for bipolar 7400 series parts, but uses CMOS technology.

Sub-types

Successive generations of technology produced compatible parts with improved power consumption, switching speed or both. Although vendors uniformly marketed these various product lines as TTL with Schottky diodes, some of the underlying circuits, such as used in the LS family, could rather be considered DTL.

Variations of and successors to the basic TTL family, which has a typical gate propagation delay of 10ns and a power dissipation of 10mW per gate, for a power-delay product (PDP) or switching energy of about 100 pJ, include:

  • Low-power TTL (L), which traded switching speed (33ns) for a reduction in power consumption (1mW) (now essentially replaced by CMOS logic)
  • High-speed TTL (H), with faster switching speed than standard TTL (6ns) but significantly higher power dissipation (22mW)
  • Schottky TTL (S), introduced in 1969, which used Schottky diode clamps at gate inputs to prevent charge storage and speed switching time. These gates operated more quickly (3ns) but had higher power dissipation (19mW)
  • Low-power Schottky TTL (LS) — used the higher resistance values of low-power TTL and the Schottky diodes to provide a good combination of speed (9.5ns) and reduced power consumption (2mW), and PDP of about 20 pJ. Probably the most common type of TTL since these were used as glue logic in microcomputers; essentially replacing the former H, L, and S sub-families.
  • Fast (F) and Advanced-Schottky (AS) variants of LS from Fairchild and TI, respectively, circa 1985, with "Miller-killer" circuits to speed up the low-to-high transition. These families achieved PDPs of 10 pJ and 4 pJ, respectively, the lowest of all the TTL families.
  • Most manufacturers offer commercial and extended temperature ranges; for example Texas Instruments 7400 series parts are rated from 0 to 70°C, and 5400 series devices over the military-specification temperature range of −55 to +125°C.
  • Radiation-hardened devices are offered for space applications
  • Special quality levels and high-reliability parts are available for military and aerospace applications.
  • Low-voltage TTL (LVTTL) for 3.3-volt power supplies and memory interfacing.

Inverters as analog amplifiers

While designed for use with logic-level digital signals, a TTL inverter can be biassed to be used as an analog amplifier. Such amplifiers may be useful in instruments that must convert from analog signals to the digital domain, but would not ordinarily be used where analog amplification is the primary purpose. TTL inverters can also be used in crystal oscillators where their analog amplifcation ability is significant in analysis of oscillator performance.

Applications

Before the advent of VLSI devices, TTL integrated circuits were a standard method of construction for the processors of mini-computer and mainframe processors; such as the DEC VAX and Data General Eclipse, and for equipment such as machine tool numerical controls, printers, and video display terminals. As microprocessors became more functional, TTL devices became important for "glue logic" applications, such as fast bus drivers on a motherboard, which tie together the function blocks realized in VLSI elements.

Captive manufacture

At least one manufacturer, IBM, produced non-compatible TTL circuits for its own use; IBM used the technology in the IBM System/38, IBM 4300, and IBM 3081.

See also

Notes

References

External links

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