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Diode logic or
Diode-resistor logic (DRL), is a circuit style that uses diodes to construct
Boolean logic gates for circuits. Only non-inverting functions may be
implemented, so it is not a complete logic family.
DRL circuits are
non-restoring. For example, if an input to a TTL gate is above 2 volts, it will
be seen as a logical high. If an input is below 0.8 volts, it will be seen as a
logical low. The output of the gate will be either 5 volts or 0 volts. This
restores the logic level. DRL circuits do not do this; the output voltage is
dependant on the input voltage.
One sense where diodes are superior to transistors is in handling large currents. If voltage is neglected, diodes are good at forming OR gates for multiple current sources.
To construct an OR gate with n logic inputs, the following components are required:
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| Diode OR gate: example with two inputs |
Whenever a logic 1 is present at the anode of a diode, it forward-biases that diode, causing it to conduct. The output voltage will be approximately 0.7 volts less than the input voltage.
If a logic 0 is present at the anode of every diode, they are all reverse-biased and the resistor drives node 1 low.
To construct an AND gate with n logic inputs, the following components are required:
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| Diode AND gate: example with two inputs |
If a logic 1 is present at the cathode of every diode, then they are all reverse biased, remaining in a high-impedance state, and the output is held high by the power supply through the resistor.
If a logic 0 is present at any input, it forward-biases that diode causing the output to get to approximately 0.7 volts.
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 with Resistor-Transistor Logic and
Diode-Transistor Logic).
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.
After introduction in
integrated circuit form in 1963 by Sylvania, TTL integrated circuits were
manufactured by several semiconductor companies, with the 7400 series by Texas
Instruments becoming particularly popular. TTL became the foundation of
computers and other digital electronics. Even after much larger scale integrated
circuits made multiple-circuit-board processors obsolete, TTL devices still
found extensive use as the "glue" logic interfacing more densely integrated
components. TTL devices were originally made in ceramic and plastic dual-in-line
(DIP) packages, and flat-pack form. TTL chips are now also made in surface-mount
packages. Successors to the original bipolar TTL logic often are interchangeable
in function with the original circuits, but with improved speed or lower power
dissipation.
TTL was invented in
1961 by James L. Buie of TRW, "particularly suited to the newly developing
integrated circuit design technology," and it was originally named
transistor-coupled transistor logic (TCTL). TTL became popular with electronic
systems designers after Texas Instruments introduced the 5400 series of ICs,
with a military temperature range, in 1964 and the later 7400 series, specified
over a narrower range, and with inexpensive plastic packages 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 Eastern Bloc
(Soviet Union, GDR, Poland, Bulgaria).
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| Two-input TTL NAND gate with a simple output stage (simplified). |
TTL inputs are the emitters of a multiple-emitter transistor. This IC structure is functionally equivalent to multiple transistors where the bases and collectors are tied together. The output is buffered by a common emitter amplifier.
The main disadvantage
of TTL with a simple output stage is the relatively high output resistance at
output logical "1" that is completely determined by the output collector
resistor. It limits the number of inputs that can be connected (the fanout).
Some advantage of the simple output stage is the high voltage level (up to VCC)
of the output logical "1" when the output is not loaded.
Logic of this type is
most frequently encountered with the collector resistor of the output transistor
omitted, making an open collector output. This allows the designer to fabricate
logic by connecting the open collector outputs of several logic gates together
and providing a single external pull-up resistor. If any of the logic gates
becomes logic low (transistor conducting), the combined output will be low.
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| Standard TTL NAND with a "totem-pole" output stage, one of four in 7400 |
TTL devices consume substantially more power than equivalent CMOS devices 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 are required between the two logic families. TTL is 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 drawback is usually overcome 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 when both the upper and lower transistors are conducting, resulting in a substantial pulse of current drawn from the power 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 pinouts. For example, the 74HCT00 series provides many drop-in replacements for bipolar 7400 series parts but uses CMOS technology.
Successive generations of technology produced compatible parts with improved power consumption or 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 10 mW 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 (1 mW)
(now essentially replaced by CMOS logic)
High-speed TTL (H), with
faster switching than standard TTL (6ns) but significantly higher power
dissipation (22 mW)
Schottky TTL (S), introduced in 1969, which used Schottky diode clamps at gate inputs to prevent charge storage and improve switching time. These gates operated more quickly (3ns) but had higher power dissipation (19 mW)
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 (2 mW) and PDP of about 20 pJ. Probably the most common type of TTL, 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.
Low-voltage TTL (LVTTL) for 3.3-volt power supplies and memory interfacing.
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.
Special quality levels and high-reliability parts are available for military and aerospace applications.
Radiation-hardened devices are offered for space applications.
Complementary metal–oxide–semiconductor (CMOS) is a technology for constructing integrated circuits. CMOS technology is used in microprocessors, microcontrollers, static RAM, and other digital logic circuits. CMOS technology is also used for several analog circuits such as image sensors (CMOS sensor), data converters, and highly integrated transceivers for many types of communication. Frank Wanlass patented CMOS in 1967 (US patent 3,356,858).
CMOS is also sometimes referred to as complementary-symmetry metal–oxide–semiconductor (or COS-MOS). The words "complementary-symmetry" refer to the fact that the typical digital design style with CMOS uses complementary and symmetrical pairs of p-type and n-type metal oxide semiconductor field effect transistors (MOSFETs) for logic functions.
Two important characteristics of CMOS devices are high noise immunity and low static power consumption. Since one transistor of the pair is always off, the series combination draws significant power only momentarily during switching between on and off states. Consequently, CMOS devices do not produce as much waste heat as other forms of logic, for example, transistor-transistor logic (TTL) or NMOS logic, which normally have some standing current even when not changing state. CMOS also allows a high density of logic functions on a chip. It was primarily for this reason that CMOS became the most used technology to be implemented in VLSI chips.
The phrase "metal–oxide–semiconductor" is a reference to the physical structure of certain field-effect transistors, having a metal gate electrode placed on top of an oxide insulator, which in turn is on top of a semiconductor material. Aluminum was once used, but now the material is polysilicon. Other metal gates have made a comeback with the advent of high-k dielectric materials in the CMOS process, as announced by IBM and Intel for the 45-nanometer node and beyond.
"CMOS" refers to both a particular style of digital circuitry design and the family of processes used to implement that circuitry on integrated circuits (chips). CMOS circuitry dissipates less power than logic families with resistive loads. Since this advantage has increased and grown more important, CMOS processes and variants have come to dominate; thus the vast majority of modern integrated circuit manufacturing is on CMOS processes. As of 2010, CPUs with the best performance per watt each year have been CMOS static logic since 1976.
CMOS circuits use a combination of p-channel and n-channel metal–oxide–semiconductor field-effect transistors (MOSFETs) to implement logic gates. Although CMOS logic can be implemented with discrete devices for demonstrations, commercial CMOS products are integrated circuits composed of up to millions of transistors of both types on a rectangular piece of silicon of between 10 and 400mm2.
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| CMOS Inverter |
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| CMOS NAND Gate |
More complex logic functions such as those involving AND and OR gates require manipulating the paths between gates to represent the logic.
CMOS dissipates power only when switching ("dynamic power"). On a typical ASIC in a modern 90-nanometer process, switching the output might take 120 picoseconds, and happens once every ten nanoseconds. Static CMOS gates are very power efficient because they dissipate nearly zero power when idle. Earlier, the power consumption of CMOS devices was not the primary concern while designing chips. Factors like speed and area dominated the design parameters. As the CMOS technology moved below sub-micron levels, the power consumption per unit area of the chip has risen tremendously.
Three-state logic gates have three states of the output: high (H), low (L) and high-impedance (Z).
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| A tristate buffer can be thought of as a switch. If B is on, the switch is closed. If B is off, the switch is open. |
When in the high-impedance state the output is disconnected internally. This allows the outputs of multiple chips to be connected to the same bus. Only the chip that is putting data on the bus is in the low-impedance state. All other chips will be in a high impedance state.
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