Transistor-transistor logic
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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). It 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. Because of the wide use of this logic family, signal inputs and outputs of electronic equipment may be called "TTL" inputs or outputs, signifying compatibility with the voltage levels used.
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[edit] History
TTL became popular with electronic systems designers in 1962 after Texas Instruments introduced the 7400 series of ICs, which had a wide range of digital logic block functions, and Sylvania introduced a similar family of products. The Texas Instrument family became an industry standard, but TTL devices are made by Motorola, Signetics, SGS-Thomson, National Semiconductor and many other companies. TTL became important because its low-cost integrated circuits made digital techniques economically practical for tasks previously done by analog methods.
The Kenbak-1, possibly the first personal computer, used TTL for its CPU instead of a microprocessor chip, which was not available in 1971. According to the inventor, the most expensive component of such a computer was memory, not the processor.[1]
[edit] Functions
Each integrated circuit performs separate building-block functions such as
- logic gates such as AND, OR, NAND, NOR, XOR, inversion
- flip-flops
- latch elements
- Ripple, synchronous, decade, and hexadecimal counters
- adders, multipliers and arithmetic logic units (ALUs)
- shift registers
- timing circuits
- data bus drivers, buffers, tristate buffers
- display drivers
- multifunction logic
- level converters
- read-write and read-only memory
- programmable logic arrays
[edit] Theory
TTL integrated circuits are examples of small-scale to large-scale integration. Each "chip" contains the equivalent of a few dozen to a few hundred transistors, contrasting with early very-large-scale integration (VLSI) devices that had the equivalent of up to 10,000 transistors, and modern microprocessors that are equivalent to tens of millions of transistors.
The fundamental switching action of a TTL gate is based on a multiple-emitter input transistor. This replaces the multiple input diodes of the earlier DTL, with improved speed and a reduction in chip area. The active operation of this input transistor removes stored charge from the output stage transistors more rapidly than a comparable DTL gate, making TTL much faster in switching. A small amount of current must be drawn from a TTL input to ensure proper logic levels. The total current draw 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 signal is defined as "low" or L when between 0V and 0.8V with respect to the ground terminal, and "high" or H when between 2V and 5V. Standardization of TTL devices was so successful that it is routine for a complex circuit board to contain chips made by many manufacturers, based on availability and cost rather than interoperability restrictions.
Like most integrated circuits of the period 1960-1990, TTL devices are usually packaged in through-hole, dual in-line packages with between 14 and 24 lead wires, made usually of epoxy plastic but also commonly ceramic. Other packages included the flat-pack, used for military and aerospace applications, and beam-lead chips without packages for assembly into larger arrays. As surface-mounted devices became more common through the 1990's, most popular TTL devices were made available in these packages.
[edit] Comparison with other logic families
Generally, TTL devices consume 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 typically slower; designers can combine ECL and TTL devices in the same system to achieve best overall performance and economy. 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 output structure, doesn't have this drawback.
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.
[edit] Sub-types
Variations of the basic TTL family, which has a typical gate propagation delay of 10ns and a power dissipation of 10mW per gate, include:
- Low-power TTL, which traded switching speed (33ns) for a reduction in power consumption (1mW) (now essentially supplanted by CMOS logic)
- High-speed TTL, with faster switching speed than standard TTL (6ns) but significantly higher power dissipation (22mW)
- Schottky TTL, 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 — 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). Probably the most common type of TTL since these were used as glue logic in microcomputers.
- 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-volt power supplies and memory interfacing.
[edit] 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 Digital Equipment Corporation 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.
[edit] See also
- Resistor-transistor logic (RTL)
- Diode-transistor logic (DTL)
- Emitter coupled logic (ECL)
- Positive Emitter Coupled Logic (PECL)
- Complementary Metal Oxide Semiconductor (CMOS)
- Integrated injection logic (I2L)
- For a more general discussion, see Digital circuit
[edit] References
- Jacob Millman, "Microelectronics Digital and Analog Circuits and Systems", McGraw-Hill Book Company, New York, 1979 ISBN 0-07-042327-X
- 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
- Fairchild Semiconductor, "Application Note 368" (for relative ESD sensitivity of TTL and CMOS)
