Structural logic analysis of MOS three-variable decoder

In computers and other digital systems, various decoders are commonly used. For example, data multiplexing control, digital display and memory addressing, etc., all use decoders.

Three-variable decoder

Figure 3-15 shows a multi-channel data transmission control system, the MOS tube in the figure serves as the transmission gate. If you want to transfer the data one by one, you need to use eight The control lines respectively control the grids of the eight gates. But using decoder control, as long as three wires are enough. Because three binary variables can form eight states, namely . MOS three-variable decoder

       Each state can control one way. We require that when a channel is selected, the corresponding output terminal of the decoder is 1, and the rest are 0. We respectively use Q, R, S, T, U, V, W, and X as the output terminals of the decoder, and use A, B, and C as the input terminals of the decoder. According to the requirements listed above, a truth table such as Table 8-3 can be written.

According to the truth table, you can write a logical formula. Since parallel gates are superior to series gates when composing a circuit, we use Morgan's theorem to turn the AND gate into a NOR gate. MOS three-variable decoder

The logic diagram and circuit are shown in Figure 3-16.

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