Page 206 - Bebop to The Boolean Boogie An Unconventional Guide to Electronics Fundamentals, Components, and Processes

Programmable ICs 18 7

Using Outputs as Inputs
To further increase the fun, some devices are constructed in such a way that
their tri-state outputs are fed back into the AND array (Figure 16-13).
a b C

yy x x ww

aa b b c c

enable
ctive Low)
Programmable Link

W X Y
Figure 16-1 3. PLA with tri-state outputs fed back as inputs

Once again, there are additional programmable links that have been
omitted from Figure 16-13 for reasons of clarity. By means of these links, each
output’s ri-state buffer can be individually configured so as to always drive
a high-impedance Z state. Pins that are configured in this way can be used
as additional inputs (Figure 16-14).
In this example, the unprogrammed device began with three inputs and
three outputs, but the user actually required a device with four inputs and only
two outputs. By means of the appropriate links, the tri-state buffer on the y
output was disconnected from the enable control and connected to a constant
logic I, Because w and x are still required to function as outputs, all of their
associated links in the AND array must be blown away to ensure that these
pins will. not have any effect as inputs. The ability to configure pins as outputs
or inputs provides a great amount of flexibility; instead of purchasing myriad
devices with every conceivable combination of inputs and outputs, the designer
can configure a single device type as required.