328 CHAPTER 7 / PROGRAMMABLE LOGIC DEVICES
longlines only via tri-state drivers. Programmable interconnect points between longlines
and single-length lines are available. However, no interconnect points are provided between
double-length lines and others.
Logic cell arrays (LCAs), such as the XC4000 series FPGAs, have the unique property
that they can be reconfigured within a system and can even be dynamically altered to perform
different functions at different times in a process. This family of devices contain 16 ×1 and
32 ×1 static RAM primitives that are user-configurable via look-up tables. Self-diagnosis,
hardware for different environments, or dual-purpose applications — these exemplify the
versatility that is inherent in reconfigurable LCAs.