Transistor-transistor logic
The transistor-transistor logic ( TTL ) is a circuit technology ( logic family ) for logic circuits ( gates ) in which planar npn bipolar transistors are used as the active component of the circuit . In the case of the N family, a multiemitter transistor is usually used at connected inputs , so that only one transistor is required for several inputs ; With the LS family and Schmitt trigger inputs, the AND interconnection of the inputs is implemented using diodes.
history
The TTL technique was invented in 1961 by James L. Buie at TRW . The first commercial circuits were produced by Sylvania Electric Products . A further development based on bipolar transistors with lower power consumption is the integrated injection logic (I²L).
Standard TTL
Standard TTL circuits are designed for operation on a supply voltage of 5 V with a deviation of 5%.
The load capacity of the outputs is referred to as fan-out , which expresses how many inputs an output can serve. This is important for the typical extensive logic circuits of the TTL age.
A high voltage is defined as a high level (in positive logic a logical 1), a low voltage is defined as a low level (in positive logic a logical 0). The circuits must be dimensioned so that input voltages U E <0.8 V are recognized as low level and U E > 2.0 V as high level. The output voltage U A at nominal load must be <0.4 V for the low level and> 2.4 V for the high level. The static signal-to-noise ratio is 0.4 V for both high and low levels. The actual output voltage is between a good 3 and just under 4 V, depending on the load.
Logical modules in TTL technology have the advantage over CMOS modules that they are less sensitive to electrostatic discharges ( ESD ). Because of the current-controlled transistors, TTL has a particularly high current consumption compared to CMOS.
The picture on the right shows the structure of a TTL NAND gate. V 1 is the multiemitter transistor, U 1 and U 2 are the inputs. A special feature of TTL circuits is that inputs deliver a small current when they are low or connected to ground. Therefore unconnected inputs are at high level. However, good practice is to still set unused inputs to high potential so that a defined or permitted potential is enforced. Unconnected inputs can massively worsen the passive interference immunity of a circuit.
functionality
The NAND gate shown opposite with 2 inputs (1/4 of a 7400 ) works as follows:
The AND function is formed by the multiemitter transistor V 1 . If input U 1 or U 2 are set to a low level (or ground), V 1 becomes conductive, since a base current now flows through R 1 . The following amplifier consists of the control transistor V 2 and a push-pull output stage ( totem pole circuit ). The base of V 2 is pulled almost to ground (U V1Sat ) by the collector of V 1 , whereby V 2 blocks. This makes the base of V 3 high and that of V 4 low . So V 3 conducts and sets the output to high . If both inputs are high , then V 2 is supplied with current via the base-collector path from V 1 and is conductive. V 3 is blocked and V 4 is conductive. The output is only low when both inputs are high - the negated AND logic function.
In the open collector version (open collector) V 3 is missing , so the collector of V 4 is led open to the output. In this case, an external pull-up resistor must be connected in place of R 3 in order to achieve a high level. This design makes it possible to connect several outputs in parallel to a "Wired-AND" (wired AND). Each of the parallel-connected outputs can pull the node low without being influenced by the logic states of the other outputs. Tristate outputs, on the other hand, may only be switched in parallel if they never have different logic states.
variants
Low power Schottky TTL
To prevent the saturation of the transistors, Schottky diodes can be connected in parallel in the base-collector path so that the voltage at the base can never be more than 0.3 V above the collector potential and only that much current flows into the base flows as necessary. That makes a Schottky transistor . The unsaturated transistors are faster (no superfluous charge carriers have to be extracted before the transistor switches off) or transistors with a higher current gain can be used and the entire circuit can be implemented with a higher resistance (and therefore more energy-saving) (S series for Schottky and LS for Low Power Schottky).
Low voltage TTL
Low-Voltage-TTL (LVTTL) is a special form of transistor-transistor logic (logic family) in which the supply voltage is reduced from 5 V to 3.3 V.
| symbol | parameter | min | Max |
|---|---|---|---|
| U IH | High-level input voltage | 2 V | U DD + 0.4V |
| U IL | Low-level input voltage | −0.4 V | 0.8V |
| U OH | High-level output voltage | 2.4V | 5 V |
| U OL | Low-level output voltage | 0 V | 0.4V |
Old types and their names
Standard TTL ICs can be recognized by a designation in the form 74xx or 74xxx , where "74" refers to the logic family and xx / xxx to the gate type (e.g. xx = "00" corresponds to NAND ). Most of the modules are also available as 54xx for the military temperature range or as 84xx for the industrial temperature range. The less popular 49xx series and the 75xx series, which primarily include interface, level converter and other matching circuits , are also built in TTL technology .
The designations of the variants are generally based on the standard type to which the module is connection and function-compatible, the variant being identified by inserted letters. The supply voltage range and the signal level are not necessarily compatible. In addition to the previously mentioned, there are numerous other TTL variants, for example:
- 74L: Low-Power TTL with lower power consumption and lower switching speed
- 74H: High-Speed TTL with much higher switching speed with higher power consumption
- 74S: Schottky TTL with higher switching speed and higher power consumption
- 74F: Almost Schottky
- 74LS: Low power Schottky
- 74AS or 74ALS: Advanced Schottky
Current types
Common TTL modules have the designation 74nn (n) xx (xx), where n is the logic family (technology) and x encodes the component function. Type 74xx blocks are out of date. Only the 74LS (low-power-schottky-successor of the 74er logic family) and 74F ( almost Schottky) are produced. Successors are in CMOS technology:
- 74HC: current CMOS family: HC stands for High Speed CMOS , supply voltage 2… 6 V, max. Frequency approx. 25 MHz
- 74AHC, 74AC: much faster than 74HC (A stands for advanced )
- 74HCT: similar to 74HC, but TTL compatible, supply voltage 4.5… 5.5 V
- 74ACT: similar to AC, but TTL compatible
The 4000 series , introduced in 1970, was the first CMOS logic family, energy-saving compared to TTL, but slow (cutoff frequency at 1 MHz) and not pin and logic compatible with TTL (supply voltage 3 to 15 V). It has a different designation key.
Related logic families
The resistor-transistor logic and the diode-transistor logic can be regarded as precursor logic families of the TTL family . These two logic families are out of date and have practically no meaning today.
Closely related to the TTL family is the slow fail-safe logic that was used in the past for specialty applications. Today this logic family also has practically no meaning any more.
literature
- Klaus Wüst: Microprocessor technology. 2nd updated and expanded edition, Friedr. Vieweg & Sohn Verlag | GWV Fachverlage GmbH, Wiesbaden 2006, ISBN 978-3-8348-0046-6 .
Web links
- www.datasheetcatalog.net (directory of data sheet collections for electronic components)
Individual evidence
- ↑ Patent US3283170 : Coupling Transistor Logic and Other Circuits. Published November 1, 1966 , inventor: J. Buie.
- ↑ 1963 - Standard Logic Families Introduced. The Computer History Museum, accessed February 17, 2010.