Page 129 - Bebop to The Boolean Boogie An Unconventional Guide to Electronics Fundamentals, Components, and Processes

110 w Chapter Eleven

An actiwe-low signal is one whose active state is considered to be logic 0.2 The
active-low nature of this particular function is also indicated by the bobbles
(small circles) associated with the symbol’s outputs and by the tilde (“-”)3
characters in the names of the output signals. Additionally, from our discussions
in Chapters 9 and 10, we know that as each output is 0 for only one input
combination, it is simpler to extract the equations in product-of-sums form.
Larger decoders are also commonly used in designs: for example, 3:8 decoders
with three select inputs and eight outputs, 4:16 decoders with four select inputs
and sixteen outputs, etc.

Tri-State Functions
There is a special category of gates called tri-state functions whose outputs
can adopt three states: 0, 1, and Z. Lets first consider a simple tri-state buffer
(Figure 1 1-6).
The tri-state buffer’s symbol is based on a standard buffer with an additional
2r
control input known as the enable. The active-low nature of this particular

-enable

Y

0 0 0 NOR
0 1 1
1 ? Z V55 (Logic

Figure 1 1-6. Tri-state buffer with active-low enable

2 Similar functions can be created with active-high outputs, which means that when an output is
selected it is asserted to a logic 1.
3 The tilde I-’ characters prefixing the output names -y[3], -y[2], -y[l], and -y[O] are used to
indicate that these signals are active-low. The use of tilde characters is discussed in more
detail in Appendix A.