Decoder

1-out-of-n decoder

A 1-out-of-n decoder is a circuit with n outputs and ld n inputs. Outputs y_J are numbered from 0 to (n — 1). An output therefore goes to "one" precisely when the input binary number A is identical to the number J of the relevant output. Figure 1.16 shows the truth table for a l-out-of-4 decoder. The variables a₀ and a_l represent the straight binary code of the number A. The sum of the products (disjunctive normal- form) of the receding functions can be taken directly from the truth table. Figure 1.17 shows the corresponding implementation.

Fig. 1.16 - Truth table of a l-out-of-4 decoder. Fig. 1.17 - Circuit of a l-out-of-4 decoder.

When using monolithic integrated circuits, hand functions are often chosen rather than and functions, so that most of the output variables are comp­lemented (barred).

Types of 1C: TTL CMOS

10 outputs 74LS42 4028

For further 1C types, see the following section on demultiplexers.

Priority decoder

The 1-out-of-n code can be converted to straight binary code by using a priority decoder. At its outputs a straight binary number appears which corresponds to the highest input number which is logic 1. The value of the lower-index input variables is irrelevant, hence the name priority decoder. This property enables the circuit to convert not only the 1-out-of-n code but also a sum code in which not just one variable is 1, but also all the less significant bits. The truth table of the priority decoder is shown in Fig.1.18.

Fig. 1.18 - Truth table of a priority decoder, x = any.

1C types:

l-out-of-10 code: SN 74147 (TTL) MC 10165 (ECL);

l-out-of-8 code, extendable: SN 74148 (TTL); MC14532(CMOS)

Date: 2015-01-12; view: 1096