Page 205 - Bebop to The Boolean Boogie An Unconventional Guide to Electronics Fundamentals, Components, and Processes

186 4 Chapter Sixteen

Additional Programmable Options -
If you thought the above was cunning, just wait From OR Array
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until you see some of the really clever tricks that i i i I
can be used to augment PLDs and make them even
more useful.

Tvi-stateable Outputs
Many PLDs have tri-state
buffers on their outputs.12 All of
the buffers share a common enable
control, which therefore requires w Y
only a single input pin on the device X
(Figure 16-11). Figure 16-1 1, PLD with
tri-stateable outputs
were omitted from Figure 16-1 1 for reasons of clarity), -
Some devices may contain additional
programmable links associated with the
outputs. By means of these additional links (which
From OR Array

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each output can be individually programmed to be
either permanently enabled or to remain under the
control of the enable input as
required (Figure 16-12).
In this example, the tri-state enable
(Active Low)
buffer driving the x output has ~~~~
been disconnected from the
enable control and connected to a

constant logic O (as these tri-state W X Y
buffers have active-low control
Figure 16-1 2. PLD with individually
inputs-as is indicated by the tri-stateable outputs
bobbles on their symbols-a logic 0
enables them while a logic 1 disables them). As a result, the x output is
permanently enabled, irrespective of the value on the enable input.

12 Tri-state buffers were introduced in Chapter 11.