Data Multiplexors - CS 61C Course Notes

1Learning Outcomes¶

Draw an n-bit wide, k-to-1 mux circuit.
Explain how the mux uses its control signal to select its output from a set of data inputs.

🎥 Lecture Video

Last time we saw how to represent and design combinational logic blocks. In this section we will study a few special logic blocks; data multiplexors, a adder/subtractor circuit, and an arithmetic/logic unit.

2The Mux¶

A data multiplexor, commonly called a mux or a selector, is a circuit that selects its output value from a set of input values. Below are two mux circuits.

"TODO" — Figure 1:A 1-bit wide, 2-to-1 MUX.

Both of these muxes have two data inputs and one output. Additionally, each mux has a special control signal labeled s, for select. The s signal is also input, but it is used to control which of the two input values is directed to the output.

Figure 1 shows a 1-bit wide, 2-to-1 mux circuit:

2-to-1 because it takes two data inputs a and b and outputs one of them.
It is 1-bit wide because all data signals (a and b) are 1-bit in width.
Notice, however, that the s signal is a single bit wide. This is because it must choose between the 2 inputs.

Figure 2 shows an n-bit wide, 2-to-1 mux circuit:

2-to-1 because it takes two data inputs A and B and outputs one of them.
It is 1-bit wide because all data signals (A and B) are 1-bit in width.
The s signal is still a single bit wide because it must choose between the 2 inputs.

The function of, say, the 1-bit wide 2-to-1 mux can be described with two rules:

\texttt{y} = \begin{cases} \texttt{a} & \text{when } \texttt{s} = 0 \\ \texttt{b} & \text{when } \texttt{s} = 1 \\ \end{cases}

To remind us of which value of s corresponds to which input, within the mux symbol we commonly label each input with its corresponding s value.

Show Answer

4 input signals
32 bits wide
2 selector bits

Muxes find common use within the design of microprocessors, e.g., those that implement RISC-V.

3MUX: Implementation¶

In most applications, you will have access to a mux; you will not need to build your own from scratch. Nevertheless, it is good to remember that like all combinational logic blocks, the function of muxes can be described using a truth table and thereby implemented as a logic gate circuit.

Click to show the gate diagrams of two muxes: a 1-bit wide, 2-to-1 mux, and a 1-bit wide, 4-to-1 mux.

1-bit wide 2-to-1 mux

Table 1:Truth table for the 1-bit 2-to-1 mux in Figure 1. Note that because there are three 1-bit inputs (a, b, and control s), the truth table has $3^2 = 8$ rows.

s	ab	y
0	00	0
0	01	0
0	10	1
0	11	1
1	00	0
1	00	0
1	01	1
1	10	0
1	11	1

To come up with the logic equation and the associated gate-level circuit diagram we can apply the technique that we studied last chapter. We write the sum-of-products canonical form and simplify through algebraic manipulation:

\begin{aligned} y &= \overline{a} \overline{b} \overline{c} + \overline{a} \overline{b} c + a \overline{b} \overline{c} + a b \overline{c} && \text{Sum of Products} \\ &= \overline{a} \overline{b} (\overline{c} + c) + a \overline{c} (\overline{b} + b) && \text{Distributivity} \\ &= \overline{a} \overline{b} (1) + a \overline{c} (1) && \text{Inverse (OR) } x + \overline{x} = 1 \\ &= \overline{a} \overline{b} + a \overline{c} && \text{Identity (AND) } x \cdot 1 = x \end{aligned}

Intuitively this result makes sense; When the control input, s, is a 0, the right hand side of the equation reduces to a, and when it is a 1, the expression reduces to b.

Figure 4:Gate diagram for a 1-bit 2-to-1 mux.

1-bit wide 4-to-1 mux

Often times we find the need to extend the number of data inputs of a multiplexor. For instance consider a 4-to-1 multiplexor in Figure 5:

Figure 5:A 1-bit wide 4-to-1 MUX.

\texttt{e} = \begin{cases} \texttt{a} & \text{when } \texttt{S} = 00 \\ \texttt{b} & \text{when } \texttt{S} = 01 \\ \texttt{c} & \text{when } \texttt{S} = 10 \\ \texttt{d} & \text{when } \texttt{S} = 11 \\ \end{cases}

How would we come up with the circuit for this mux?

Approach 1. We could start by enumerating the truth- table—in this case the function has 4 single bit data inputs and one 2-bit wide control input, for a total of 6 single bit inputs. The truth table would have 26, or 64 rows. Certainly, a feasible approach. If we where to do this, we would end up with the following logic equation:

e = \overline{s_1 s_0} a + \overline{s_1} s_0 b + s_1 \overline{s_0} c + s_1 s_0 d

Approach 2. Another way to design the circuit is to base it on the hierarchical nature of multiplexing. We can build a 4-to-1 mux from three 2-to-1 muxes as shown in Figure 6. The first layer of muxes uses the $s_0$ input to narrow the four inputs down to two, then the second layer uses $s_1$ to choose the final output.

Figure 6:4-to-1 multiplexor (MUX) circuit diagram.