Lattice GAL16V8D-15LD: Architecture, Programming, and Application in Digital Logic Design
The Lattice GAL16V8D-15LD stands as a quintessential example of a Generic Array Logic (GAL) device, a pioneering technology that helped bridge the gap between expensive, mask-programmable PALs and the inflexibility of standard logic ICs. As a 15ns high-speed, low-power CMOS device in a 20-pin plastic leaded chip carrier (PLCC) package, it offered designers a reconfigurable and reliable solution for implementing complex combinational and sequential logic circuits. Its architecture, programming methodology, and practical utility cemented its role as a foundational component in digital systems throughout the late 1980s and 1990s.
Architecture: A Flexible AND-OR Structure
The internal architecture of the GAL16V8D is ingeniously designed around a programmable AND array feeding into a fixed OR array. This structure is the core of its Sum-of-Products (SOP) logic capability. The "16V8" designation is descriptive: the device has up to 16 dedicated input pins and 8 output pins, each of which can be configured as either an input or an output, providing significant I/O flexibility.
A key architectural advancement over its PAL predecessors was the inclusion of Output Logic Macro Cells (OLMCs). Each of the eight outputs is associated with an OLMC, which can be programmed—via a set of architecture control bits—to operate in different modes: registered (with a D-type flip-flop) or combinatorial. This allows the same device to implement both simple glue logic and more complex state machines. The programmable electronic fuses (E²CMOS technology) within the AND array allow patterns of connections to be made, defining the Boolean logic functions. The `-15LD` suffix specifically denotes a 15ns maximum propagation delay and a PLCC package.
Programming: From Boolean Equations to JEDEC File
Programming the GAL16V8D is a hardware-agnostic process centered on software tools. A designer defines the desired logic functionality using Hardware Description Languages (HDLs) like Abel-HDL or Cupla, or more commonly through schematic capture or Boolean equations. These tools then perform logic minimization and translate the design into a fuse map file in the JEDEC standard format.
This JEDEC file is physically transferred to the GAL device using a dedicated programmer or gang programmer. The programmer applies specific voltages to the chip, selectively blowing the internal fuses to create the desired circuit pattern. A critical advantage of the E²CMOS technology is its reprogrammability, allowing the device to be erased and reprogrammed thousands of times, which greatly accelerated design prototyping and debugging cycles.
Application in Digital Logic Design
The primary application of the GAL16V8D-15LD was to replace multiple standard TTL logic chips (like the 7400 series) with a single, integrated component. This led to significant benefits in board space reduction, improved reliability (fewer solder joints and components), and enhanced design security (the programmed function is not easily reverse-engineered).
Its speed made it suitable for implementing high-speed state machines, address decoders, bus interfaces, and complex combinatorial logic. For instance, it could be programmed to create a multiplexer, a counter, or a custom waveform generator, consolidating what would have required a handful of simpler ICs. It served as the "glue logic" that interconnected larger, more complex components like microprocessors and memory chips, tailoring their interaction to the specific needs of the system.
The Lattice GAL16V8D-15LD was a workhorse of its era, embodying the shift towards programmable logic. Its blend of speed, flexibility, and reprogrammability made it an indispensable tool for engineers, reducing development time and cost while increasing system integration. It paved the way for the more complex CPLDs and FPGAs that dominate today, representing a critical milestone in the history of digital logic design.
Keywords:
Programmable Logic Device (PLD)
Generic Array Logic (GAL)
Output Logic Macro Cell (OLMC)
JEDEC file
Sum-of-Products (SOP)
