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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Logic elements from the inside. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Beginner radio amateur Digital microcircuits are designed for processing, converting and storing digital information. They are issued in series. Within each series, there are groups of devices united by functionality: logical elements, triggers, counters, elements of arithmetic devices (performing various mathematical operations), etc. The wider the functional composition of the series, the greater the capabilities of a digital device made on the basis of microcircuits of this series. The microcircuits included in each series have a single design and technological design, a single supply voltage, the same levels of logic 0 and logic 1 signals. All this makes the microcircuits of the same series compatible. The basis of each series of digital microcircuits is the basic logic element. As a rule, basic logical elements perform AND-NOT or OR-NOT operations and, according to the principle of construction, are divided into the following main types: elements of diode-transistor logic (DTL). resistive-transistor logic (RTL), transistor-transistor logic (TTL), emitter-coupled transistor logic (ESTL), microcircuits on the so-called complementary MIS structures (CMDP). KMDP elements of digital microcircuits use pairs of MIS transistors (with a metal-dielectric-semiconductor structure) - with p- and n-type channels. Basic elements of other types are made on bipolar transistors. In amateur radio practice, TTL and KMDP series microcircuits are most widely used. On (Fig. 1) shows a diagram of the basic logic element AND-NOT TTL. At the input of the element, a multi-emitter transistor VT1 is turned on. If high voltages are applied to all of its emitters, then the emitter junction of the transistor will be closed. At the same time, the current flowing through the resistor R1 and the collector junction of the transistor VT1 will open the transistor VT2. The voltage drop across the resistor R3 will be sufficient to open the transistor VT5. The voltage at the collector of the transistor VT2 is such that the transistor VT3 is closed, and the transistor VT4 is closed accordingly. As a result, a low level voltage corresponding to logic 0 will appear at the output of the element. If, however, a low-level voltage is applied to at least one of the inputs of the element, then the emitter junction of the transistor VT1 will open, and the transistors VT2 and VT5 will be closed. The VTZ transistor will open due to the current flowing through the resistor R2 and enter saturation mode. Accordingly, the VT4 transistor will open, and a high-level voltage corresponding to logical 1 will appear at the output of the element. Therefore, the considered element performs the NAND function. The composition of TTL series microcircuits also includes a AND-NOT logic element without a collector load in the output stage. This is the so-called open-collector NAND element. It is designed to operate on an external load, which can be used as electromagnetic relays, indicator devices, etc., open-collector circuits are also used in data buses in cases where two or more outputs are connected to one physical line (Figure 1).
Recall that the CMDS structure is an ideal voltage switch. Such a switch contains two MIS transistors with p- and n-type channels. When a high level voltage is applied to the input of the switch, the n-channel transistor opens and the p-channel one closes. Figure 2 shows the diagrams of the basic elements of AND-NOT (a) and OR-NOT (b) of KMDP microcircuits. A low-level voltage (logical 0) will be at the output of the AND-NOT element only if high-level voltages (logical 1) are simultaneously applied to all inputs X1-X1. If the voltage at at least one of the inputs (for example, X6) is low, then the n-channel transistor VT1 will close, and the p-channel transistor VTXNUMX will open, through the channel of which the output of the element is connected to the power source. Thus, the output will have a high-level voltage corresponding to logic 1. To implement the basic OR-NOT logic element on CMOS structures, circuit sections containing transistors connected in series and in parallel should be swapped in Figure 2,b.
TTL microcircuits are designed for a power supply voltage of 5 V ± 10%. Most microcircuits based on CMOS structures operate stably at a supply voltage of 3-15 V, some - at a voltage of 9 V ± 10%. Logic levels 0 and 1 should differ as much as possible. There are threshold logic 1 U1thor - the smallest high-level voltage at the input of the microcircuit, at which the output voltage changes from the level of logical 0 to the level of logical 1, as well as the threshold voltage of logical 0 U0thor - the highest low-level voltage at the input of the microcircuit, at which the output voltage changes from the level of logical 1 to the level of logical 0. Before proceeding to a detailed consideration of the most common series of microcircuits and digital devices based on them, let us dwell on the main parameters of logic elements. These include power supply voltage, logic 0 and logic 1 voltage levels, load capacity, noise immunity and speed, power consumption. For TTL microcircuits of the U1por series = 2,4 V; U0por \u0,4d 1 V. The voltage of low and high levels at the output of TTL microcircuits U2,4out>=1V, U0,4out<=XNUMXV. For microcircuits based on CMOS structures U1por>0,7* Upit, U0por>0,3* Upit, the same time of deviation of the output voltages U0out and U1out from zero and the power supply voltage, respectively, reach only a few tens of millivolts. The ability of an element to work on a certain number of inputs of other elements without additional matching devices is characterized by a load capacity. The higher the load capacity, the fewer elements may be needed when implementing a digital device. However, with an increase in the load capacity, other parameters of microcircuits deteriorate: speed and noise immunity decrease, and power consumption increases. In this regard, as part of various series of microcircuits, there are so-called buffer elements with a load capacity that is several times greater than that of the main elements. Quantitatively, the load capacity is estimated by the number of unit loads that can be simultaneously connected to the output of the microcircuit. In turn, a single load is the input of the main logic element of this series. The output branching factor for most logic elements of the TTL k155 series is 10, for k561 KMDP series microcircuits it is up to 100. The noise immunity of basic logic elements is evaluated in static and dynamic modes. In this case, static noise immunity is determined by the level of voltage applied to the input of the element relative to the levels of logical 0 and 1, at which the state at the output of the circuit does not change. For TTL elements, static noise immunity is at least 0,4 V, and for microcircuits of the KMDP series, at least 30% of the supply voltage. Dynamic noise immunity depends on the shape and amplitude of the noise signal, as well as on the switching speed of the logic element and its static noise immunity. The dynamic parameters of the basic elements are evaluated, first of all, by their speed. Quantitatively, the performance can be characterized by the limiting operating frequency, i.e., the maximum switching frequency of the trigger performed on these basic elements. The limiting operating frequency of TTL chips of the k155 series is 10 MHz. and microchips of the k176 and k561 series on CMDP structures are only 1 MHz. The performance is defined in the same way as the average signal propagation delay time.
tsp.r.av.=0,5(t1,0zd.r+t0,1zd.r), where t1,0zd.r and t0,1zd.r - signal propagation delay times when switching on and off Figure 3. The average signal propagation delay time is a more universal parameter of microcircuits, since, knowing it. it is possible to calculate the speed of any complex logic circuit by summing tzd.r.sr for all series-connected microcircuits. For microcircuits of the K155 series, tsp.r.sr is about 20 ns, and for microcircuits of the K176 series - 200 ns. The power consumed by the microcircuit in static mode turns out to be different at the levels of logical zero (P0) and logical one at the output (P1). In this regard, the average power consumption Рav=(Р0+Р1)/2 is measured. The static average power consumption of the basic elements of the K 155 series is several tens of milliwatts, while for the elements of the K176 and K561 series it is more than a thousand times less. Therefore, if it is necessary to build digital devices with a low current consumption, it is advisable to use microcircuits based on CMOS structures. However, it should be borne in mind that when operating in dynamic mode, the power consumed by the logic elements increases. Therefore, in addition to Рср, the power Рdyn, measured at the maximum switching frequency, is also set. Must be borne in mind. that with increasing speed, the power consumed by the microcircuit increases Author: -=GiG=-, gig@sibmail; Publication: cxem.net
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