Page 208 - Bebop to The Boolean Boogie An Unconventional Guide to Electronics Fundamentals, Components, and Processes

Programmable ICs 189

flops, but alternative register types such as JK flip-flops or T-type flip-flops may

be more suitable for certain applications. It can be inconvenient to support a
dedicated device for each type of register. As a solution, some devices have
configurable register
dff=D-type flip-flop rn ux
elements whose types can
be selected by prograrn- w
appropriate fuses.
egistered (or latched)
outputs may also incorpo- From ’ X
rate by-pass multiplexers OR *rray
(Figure 16-16).
By means of appropri-
Y
ate fuses, the control
inputs to the multiplexers
can be individually
(positive edge)
configured to select
either the non-registered data Figure 16-1 6. PLD with registered
outputs and by-pass multiplexers
or its registered equivalent+
Other common programmable options are the ability to select true or
complemented outputs and TTL- or CMOS-compatible output drivers.
An individual PLD typically only provides a subset of the above capabilities,
but these may be combined in a variety of ways; for example, registered outputs
may be followed by tri-state buffers.

Programming PLDs
Programming a traditional PLD is relatively painless because there are
computer programs and associated tools dedicated to the task. The user first
creates a computer file known as a PLD source file containing a textual
description of the required functionality (Figure 16- 17).
In addition to Boolean equations, the PLD source file may also support
truth tables, state tables, and other constructs, all in textual format. The
exclamation marks (called shrieks) shown in these equations provide a textual
way of indicating inversions. Additional statements allow the user to specify
which outputs are to be tri-statable, which are to be registered, and any of the
other programmable options associated with PLDs.