Digital Electronics

n digital systems, a limited number of circuit states, usually two in the nearly universal binary logic, contain the information, instead of the continuously variable information in an analog signal. The science of digital logic is presented in texts and courses in Digital Design, to which the reader is referred for a great deal of interesting information, most of which is not used in practical work, at least not any more. Here, I want to discuss mainly the electronic or circuit aspects of the integrated circuits used in digital devices, which is of great utility in practical work. We will not be very interested in the information carried by the digital signals.

There are two kinds of digital integrated circuits in common use. There are more varieties, some quite esoteric, but these two handle most of the load, and are the only ones easily available for study and use. The two kinds are bipolar (“TTL”) and CMOS, and both are about as old as integrated circuits themselves. Their circuit properties are quite different, although modern devices can actually be used together (this is not usually a good practice, however). Bipolar devices, somwhat inappropriately called TTL (transistor-transistor logic), early became the most popular choice, together with NMOS large-scale devices that imitated TTL properties. CMOS with its rugged flakiness was used when power drain had to be kept to a minimum, and for certain devices where its peculiarities were valuable, as in ripple counters and the 4046, for example. The TTL integrated circuits were given numbers 74XX, while the CMOS circuits had numbers 4XXX.

The two states of TTL logic were a low state near 0 V produced by a saturated transistor switch, and a high state of rather indefinite voltage, provided either by a pull-up resistor, or by the “totem-pole” output of a circuit, which pulled up the output near 5 V. These two states are very easily distinguished, and insensitive to noise, especially the low state. The effect of noise depends not only on the voltage outputs, but on the output resistance as well, so comparison of voltage levels is not a reliable guide. 4000 CMOS suffered from high output resistance and high input resistance as well, which allowed small extraneous charges to cause problems. However, the saturated transistors of TTL took a lot of current, and also a lot of time to get out of saturation (though they were much faster than CMOS).

TTL was modified by a Schottky diode between collector and base that prevented transistors from going into deep saturation, saving both current and time. The circuits were also redesigned to use less current, and were called LS, or low-power Schottky. The numbers were now 74LSXX. There have been further developments (e.g., “fast” or 74FXX, “low-power” or 74LXX, “Schottky” or 74SXX–same power as TTL but much faster, etc.) but LS is excellent, inexpensive and superior for general use.

CMOS was also greatly improved by giving the outputs a lower resistance, and making the circuits work more like TTL. The result was the 74HCXX series, which has the same functions as TTL, even the same pinouts, and can replace LSTTL in most applications. There is a variation in which the inputs are made to mimic TTL inputs, called HCT, with part numbers 74HCTXX. These work reliably when LSTTL outputs are connected to their inputs, while HC might not. Otherwise, they have no advantage over the normal HC chips, which should usually be chosen. Although HC chips resemble TTL in look and function, they are true CMOS, with the peculiarities of that family.

HC (or HCT) outputs can drive up to 10 LSTTL inputs. Even 4000-series CMOS can drive 2 LSTTL inputs. A CMOS output swings from near the supply to near ground, so will be properly interpreted by any logic input. With a 4.5V supply, the HC input logic levels are H > 3.15 V and L < 1.35 V, whereas the LSTTL and HCT levels are 2.0 V and 0.8 V, respectively. The CMOS input current is guaranteed less than 1 μA, but is typically a tenth of that or less. Some input current is required to charge the input capacitance of about 5 pF, but this will have little effect. The arrangement and pinouts of the chips we shall exercise here are shown in the figure below. The symbols for inverters and gates will be familiar. An open circle represents logic inversion, but really a pin that is "active low," that is, performs its function when pulled low, while its normal state is high. This usually has nothing to do with digital logic, and is a pure circuit function. The power connections for all these chips is in the standard "corner" positions, pins 14 or 16 for +5, and pins 7 or 8 for GND. The 14-pin and 16-pin packages are standard for LS and HC digital logic circuits.

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