Generic Array Logic (GAL): Applications, Best Practices and Comparison with CPLD & FPGA

Introduction: The Legacy of Programmable Logic Devices

In the ever-evolving world of digital electronics, programmable logic devices (PLDs) have played a critical role in bridging the gap between custom application-specific integrated circuits (ASICs) and general-purpose microcontrollers. Among these, Generic Array Logic (GAL) stands out as a significant milestone, offering reprogrammable, compact, and cost-effective solutions for a variety of digital applications. But what exactly is GAL, and why has it been so important to the evolution of modern logic design?

To fully appreciate GAL, we must first understand its predecessors. The development of PLDs began with Programmable Logic Arrays (PLAs) and Programmable Array Logic (PAL) devices in the 1970s and 1980s. While these devices allowed engineers to program custom logic, they were limited by their one-time programmable nature and lack of flexibility. This is where GAL revolutionized the landscape.

What is Generic Array Logic (GAL)?

Generic Array Logic (GAL) is a type of programmable logic device that uses Electrically Erasable Programmable Read-Only Memory (EEPROM) technology, making it reprogrammable and reusable. Introduced by Lattice Semiconductor in the mid-1980s, GAL became an instant upgrade over PAL devices due to its flexibility and cost-effectiveness.

At its core, GAL functions as a hardware-efficient logic gate array that can be configured to perform various combinational and sequential logic operations. This reprogrammable capability allows engineers to modify designs without discarding hardware, significantly reducing development costs and time.

Key Advantages of GAL Over PAL and PLA

1. Reprogrammability:
   Unlike PAL devices, which are one-time programmable, GAL uses EEPROM technology. This allows for multiple reprogramming cycles, making it a versatile choice for prototyping and iterative design processes.

2. Pin and Function Compatibility:
   GAL devices are often designed to be pin-compatible replacements for PAL devices, making them ideal for upgrading or maintaining legacy systems without major redesigns.

3. Enhanced Reliability and Speed:
   With optimized internal architectures, GAL devices offer higher speed and reliability compared to earlier PLDs.

4. Compact and Cost-Effective:
   By integrating multiple logic functions into a single chip, GAL eliminates the need for discrete components, reducing board space and material costs.

Applications of GAL in Early Electronics

GAL devices became the go-to solution for engineers working on:

• Address decoding in microprocessor systems
• State machine implementation for control logic
• Signal routing and timing control in embedded systems
• Replacing obsolete PAL devices in legacy systems

GAL not only streamlined the process of creating digital logic but also opened the door to more sophisticated design methodologies. By reducing the time and expense associated with creating custom logic, it democratized access to advanced digital systems for smaller companies and independent designers.

The Evolution of GAL in the Digital Age

While GAL devices were groundbreaking at the time, their simplicity became a limitation as the demand for more complex systems grew. With the advent of Complex Programmable Logic Devices (CPLDs) and Field-Programmable Gate Arrays (FPGAs), GAL's role has shifted from cutting-edge technology to a niche solution for specific use cases. Yet, its impact on digital design is undeniable, laying the groundwork for reprogrammable logic that we now take for granted.

In the previous section, we explored the historical context and foundational advantages of Generic Array Logic (GAL). Now, it's time to delve deeper into the inner architecture that makes GAL devices both powerful and versatile. By understanding their internal design, we can appreciate how GAL became a cornerstone in the evolution of programmable logic.

Internal Architecture of GAL Devices

At the heart of a GAL device lies its programmable AND array and fixed OR array, which together allow for the implementation of complex logic functions. Let’s break down the core components that define this architecture:

1. Programmable AND Array
   The AND array in a GAL device is a programmable logic matrix where each input line can be connected to any output line via electrically programmable fuses (EEPROM cells). This structure allows for the generation of product terms — logical AND operations involving one or more inputs and their complements.

   For example, an input $A$ and its complement A‾A can be programmed to form terms like $A\cdot B$, A⋅C‾A⋅C, or more complex combinations.

2. Fixed OR Array
   The product terms generated in the AND array are passed into a fixed OR array, where they are combined to create the desired sum-of-products expression. This architecture adheres to the fundamental principles of Boolean algebra, making it highly flexible for implementing a variety of logic functions.

   For instance, the sum-of-products expression:

   Y=(A⋅B)+(A‾⋅C)Y=(A⋅B)+(A⋅C)

   is easily realized using this structure.

3. Output Logic Macrocells (OLMCs)
   What truly sets GAL devices apart is the inclusion of output logic macrocells. These macrocells provide the flexibility to configure each output as:

   • Combinational logic: Directly reflecting the sum-of-products result.
   • Registered output: Incorporating a flip-flop to enable sequential logic.

   Each macrocell typically includes:

   • A flip-flop for optional clocked storage.
   • An output enable control for tri-state operations.
   • Configurable feedback paths for reusing the output as an input to the AND array.

4. Feedback Pathways
   Feedback paths in GAL devices allow outputs to be routed back into the programmable AND array as inputs. This feature is crucial for implementing state machines and other sequential logic circuits.

Macrocell Configuration: The Key to Versatility

A macrocell in a GAL device can be configured in multiple ways, enabling it to function as:

• Latches and Flip-Flops: Essential for building state machines and counters.
• Multiplexers: Routing one of many inputs to a single output.
• Logic Gates: Creating custom AND, OR, XOR, or NAND gates.
• Memory Elements: Storing states in sequential circuits.

This configurability gives GAL devices an edge over earlier PLDs, as they can handle both combinational and sequential logic within the same framework.

The Programming Process

To program a GAL device:

1. Logic Design: Engineers define the desired logic using hardware description languages (HDLs) like Verilog or VHDL, or with schematic-based design tools.
2. Compilation: The design is compiled into a programming file (JEDEC format) that maps the logic to the programmable AND array.
3. Programming: A GAL programmer writes this configuration to the device’s EEPROM cells, effectively "burning" the logic into the chip.

The reprogrammable nature of EEPROM ensures that the device can be erased and reprogrammed multiple times, allowing for iterative design refinement.

Real-World Example: A 4-to-1 Multiplexer Using GAL

To demonstrate GAL’s capabilities, consider the implementation of a 4-to-1 multiplexer. The truth table for the multiplexer maps four inputs (I0I0​ to I3I3​) to a single output $Y$, based on two selection lines (S0S0​ and S1S1​).

The Boolean expression for $Y$ is:

Using a GAL device:

1. The inputs (S0S0​, S1S1​, I0I0​ to I3I3​) are mapped to the programmable AND array.
2. The terms for $Y$ are generated in the AND array.
3. The fixed OR array combines these terms to produce the final output.

Such an implementation is compact, efficient, and fully customizable — a hallmark of GAL’s design flexibility.

Benefits of GAL’s Architecture

The internal structure of GAL devices offers several key benefits:

• Speed: By minimizing the need for external components, GAL reduces propagation delays.
• Flexibility: The combination of programmable arrays and configurable macrocells supports a wide range of applications.
• Reusability: The ability to reprogram logic ensures long-term utility, even as requirements change.
  
  

Practical Applications of GAL

1. Address Decoding in Microprocessor Systems
   One of the most common uses of GAL devices is in address decoding, where they determine which memory or peripheral device a microprocessor should access. A GAL can efficiently implement complex decoding logic, offering:

   • Reduced chip count compared to discrete logic ICs.
   • Flexibility to adapt to different address maps via reprogramming.

2. Finite State Machines (FSMs)
   GAL devices excel at implementing state machines for controlling sequential processes. By utilizing their flip-flops and feedback paths, engineers can create compact FSMs used in:

   • Industrial automation systems.
   • Communication protocols.
   • Embedded system control units.

3. Glue Logic for Interfacing Components
   When integrating diverse digital components, glue logic is often needed to handle signal translations, timing adjustments, or protocol conversions. GAL devices provide a fast, customizable solution for such tasks, especially in systems with mixed-voltage or legacy components.

4. Custom Counters and Timers
   With configurable flip-flops and combinational logic, GAL devices are ideal for creating custom counters, pulse generators, and timing circuits in applications like:

   • LED displays.
   • Simple robotics.
   • Sensor data sampling.

5. Replacement for Obsolete Components
   In many legacy systems, PAL or TTL logic ICs are no longer available. GAL devices serve as drop-in replacements, providing identical functionality with the added advantage of reprogrammability, making them invaluable for maintaining old hardware.

Best Practices for Designing with GAL

To maximize the potential of GAL devices, consider the following design strategies:

1. Optimize Logic Simplification
   Use tools like Karnaugh maps or software synthesis to minimize the number of terms in your logic expressions. This reduces the use of AND gates and improves performance.

2. Plan Feedback Carefully
   Feedback paths are powerful but can introduce unexpected delays or glitches if not managed properly. Use simulation tools to verify the behavior of state machines and sequential circuits before programming.

3. Minimize Output Glitches
   When configuring combinational outputs, avoid logic conditions that might cause glitches during transitions. For example, implement proper synchronization for asynchronous signals.

4. Document Configuration and Revision History
   Since GAL devices are reprogrammable, maintaining clear records of the programmed logic and configuration settings is critical for troubleshooting and future updates.

5. Verify with Simulation
   Use HDL simulation tools to model your GAL-based design. This helps identify potential issues early and ensures compatibility with larger systems.

6. Consider Thermal Management
   Although GAL devices are power-efficient, they can heat up in high-frequency applications. Ensure adequate ventilation or heatsinking in dense designs.

The Role of GAL in the Modern Era

As digital systems grow more complex, GAL devices have largely been overshadowed by CPLDs and FPGAs, which offer higher logic density, more advanced features, and integrated design environments. However, GAL still holds relevance in several niches:

1. Educational Tools
   GAL devices are simple enough for beginners to understand and program, making them a great tool for teaching fundamental concepts of programmable logic.

2. Prototyping and Low-Cost Designs
   For small-scale projects that don’t require the power of FPGAs, GAL offers a cost-effective and straightforward solution.

3. Legacy System Maintenance
   Many industries, such as aerospace and industrial automation, rely on systems designed decades ago. GAL devices provide an easy way to maintain or upgrade these systems without overhauling their entire design.

Comparing GAL with CPLDs and FPGAs

To better understand where GAL stands today, let’s compare it with Complex Programmable Logic Devices (CPLDs) and Field Programmable Gate Arrays (FPGAs):

The Enduring Legacy of GAL

Though modern alternatives like FPGAs dominate the programmable logic landscape, GAL devices remain an important chapter in the evolution of digital design. Their simplicity, reliability, and reprogrammable nature have made them indispensable in their era and relevant in specific modern-day scenarios.

For engineers working with GAL today, the lessons learned from its architecture and design processes serve as a foundation for mastering more advanced programmable logic devices. As technology continues to evolve, the fundamental principles established by GAL will undoubtedly persist in newer forms, ensuring its legacy lives on.

Conclusion

This three-part exploration of Generic Array Logic (GAL) has highlighted its historical significance, technical brilliance, and practical versatility. From understanding its reprogrammable architecture to implementing real-world applications and comparing it with modern alternatives, we’ve journeyed through the essential aspects of this remarkable technology.