Understand the functional knowledge analysis of MOS integrated circuits from the perspective of logic design

The logic design of electronic instruments based on the use of integrated circuits (not limited to MOS integrated circuits) is much more restricted than when using so-called discrete components such as transistors and diodes. The ultimate goal of logic design is to reduce costs. Based on this alone, logic design should also be carried out from a perspective different from that of discrete components. The problem that must be paid attention to when designing is the basic logic structure and the number of the same logic function. MOS logic integrated circuit. Especially when using MOS integrated circuits, the former (basic logic structure) will take up a large proportion in the design of the whole machine. This is analyzed below.

The so-called basic logic structure method includes the method of logic structure with gate as the main body and the method of trigger structure with flip-flop as the main body. In the past, when discrete components and bipolar integrated circuits were used, a gate-based approach was used, but for MOS integrated circuits, the conditions of use are quite different. MOS logic integrated circuit. That is to say, compared with discrete components and bipolar integrated circuits, MOS integrated circuits are characterized by the fact that the price of flip-flops is cheaper than gate circuits. This shows that the functional efficiency of MOS integrated circuits is very good, and there are fewer components (MOS transistors) required to form a logic circuit. For example, a 12-lead TO-5 package can usually be packaged with 4 input terminals and a small number of output terminals. Type trigger. Therefore, when using integrated circuits with gates or flip-flops as units to compose electronic equipment, use this feature of MOS integrated circuits to use flip-flops and less gate circuits.

Here, take the timing signal generator that is indispensable in serial digital computers as an example. Figure 3.2 is a circuit composed of stacking and decoding gate circuits of binary counters used in the past. An integrated circuit containing two JK flip-flops requires two, and an integrated circuit containing two four-input AND gates requires eight. , An integrated circuit containing three three-input AND gates requires one, and a total of 11 integrated circuits are required. In contrast, if the ring counter with error correction capability shown in Figure 3.3 is used, 4 integrated circuits containing four D-type flip-flops and 2 four-input NAND gate integrated circuits are required. Constructed with 6 integrated circuits. Obviously, it is necessary to study the basic logic circuit structure and control method to adapt to MOS integrated circuits.

Let's study the problems that must be encountered when using integrated circuits, that is, the number of identical logic functions. The solid line in Figure 3.3 is a hexadecimal ring counter. Let's take a look at what happens to the 17-ary ring counter? Although you can simply add a D-type flip-flop, on the other hand, it will cause problems. The synchronization signal of an integrated circuit is common, so the synchronization signals of the 4 flip-flops in the added integrated circuit are also the same. If the flip-flop driven by this synchronization signal has no other purpose, the remaining 3 The trigger is completely redundant. MOS logic integrated circuit. In this case, it is necessary to study the method of constructing a 17-ary counter without adding an integrated circuit, and at the same time to study the use of 3 flip-flops under this condition to simplify other logic problems. The dotted line in Figure 3.3 shows how to use the four input terminals of the four-input NAND gate to form a 17-ary counter. The research on the composition method is very necessary, because it directly affects the cost. Of course, this type of problem is not limited to flip-flops, but also applies to inverters and gate circuits.

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