Multiplexers (Selectors)

The opposite of a demultiplexer is a multiplexer. Starting from the circuit in Fig. 1.43, it can be implemented by swapping the outputs and input to give the basic circuit in Fig. 1.34. This provides a particularly simple illustration of the mode of operation: a 1-out-of-n decoder selects from n inputs the one whose number coincides with the number entered and switches it to the. output. The corresponding gate implementation is shown in Fig. 1.35.

Fig. 1.34 - Basic multiplexer operation. Fig. 1.35 - Multiplexer circuit.

In CMOS technology, a multiplexer can be implemented using both gates and analog switches (transmission gates). When analog switches are employed, signal transmission is bidirectional. In this case, therefore, the multiplexer is identical to the demultiplexer, as comparison of Figs. 1.34 will show. The circuit is then known as an analog multiplexer/demultiplexer.

Fig. 1.36 - Multiplexer with open-collector gates. Fig. 1.37 - Multiplexer with tristate gates.

In order to overcome the disadvantage associated with open-collector out­puts, namely the higher switching time, Instate outputs can be connected in parallel, with only one being activated at a time. This alternative is shown in Fig. 1.37.

Although the possible implementations of the or operation shown in Figs. 1.36 and 1.37 are not employed in integrated multiplexers, they are useful if the signal sources of the multiplexer are spatially distributed. Arrangements of this kind are found in bus systems, as we shall describe in Chapter 21.

Some commonly used multiplexers are listed in Fig. 1.38.

¹ Manufacturer: MMI

Fig. 1.38 - Integrated multiplexers. "CMOS analog" means multiplexer/demultiplexer with transmis­sion gate.

A 2ⁿ-input multiplexer is a combinational system with n control (select) inputs s = (s_n-1, . . . , s₀), 2ⁿ data inputs x = ( , . . , x₀), a module enable E, and one data output y (Figure1.39). The values of the control variables s are interpreted as the binary representation of an integer from 0 to 2ⁿ -1. The output y corresponds to data input x_i, if s represents the integer i.

Fig. 1.39 - 2-input multiplexer.

A high-level description is.

where

In the description of systems, this module is denoted as

y = MUX(x, s, E)

For example, MUX((1,0,1,1), (1,0), 1) = 0.

The switching expression representing the multiplexer is

where m_i(s) is the ith minterm of the n select variables.

A vector multiplexer is a generalized multiplexer in which each of the data inputs the output are k-bit vectors. A block diagram is shown in Figure 1.40. A multiplexer is used whenever one of several objects has to be selected for transmission or as input to some combinational system.

Fig. 1.40 - Kx2-input (vector) multiplexers.

Date: 2015-01-12; view: 1593