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280 CHAPTER 6 / NONARITHMETIC COMBINATIONAL LOGIC DEVICES

EN(L)

EN
'1 'o Y 3 Y 2 Y 1 Y 0
0 X X .. .. .. ..
1 0 0 0 0 0 1
1 0 1 0 0 1 0
1 1 0 0 1 0 0
1 1 1 1 0 0 0
X = Irrelevant input

(a)

Y 3(L)

(b)
FIGURE 6.40
Transmission gate implementation of the 2-to-4 decoder, (a) Truth table, (b) Logic circuit with active
low outputs produced by tri-state buffer/enables.

circuits. It has since gone through several revisions and in 1987 and 1993 was adopted as
the IEEE 1076-1987 and 1076-1993 standards, respectively. The examples given in this
section will use these standards.
VHDL is a powerful tool capable of top-down design descriptions covering various
levels of abstraction ranging from the behavior level, to the dataflow level, to the structural