Programmable Array Logic (PALs): The Building Blocks of Custom Digital Circuits

In the world of digital electronics, flexibility and customization are key to creating efficient and specialized circuits. One of the technologies that revolutionized this field is Programmable Array Logic, commonly known as PAL. In this blog post, we’ll explore the world of PALs, their history, how they work, their applications, and their evolution in modern electronics.

What is Programmable Array Logic?

Programmable Array Logic (PAL) is a type of programmable logic device (PLD) used to implement combinational logic circuits. PALs allow engineers to create custom digital circuits by programming connections between an AND-plane and an OR-plane, providing a flexible and efficient way to implement complex logic functions.

The key feature of PALs is their ability to be programmed after manufacturing, allowing for customization and reducing the need for multiple specialized chips. This programmability makes PALs an essential tool in prototyping and small to medium-scale production runs.

A Brief History of PALs

The concept of PALs was developed in the late 1970s by John Birkner and H. T. Chua at Monolithic Memories, Inc. (MMI). The first PAL device, the 16L8, was introduced in March 1978.

Key milestones in PAL history include:

- 1978: Introduction of the first PAL device (16L8)
• 1983: Advanced Micro Devices (AMD) acquired MMI
• 1985: Introduction of the 22V10, one of the most popular PAL devices
• Late 1980s: Development of more complex PLDs, leading to CPLDs and FPGAs

PALs quickly gained popularity due to their flexibility and ease of use compared to discrete logic components, becoming a staple in electronic design throughout the 1980s and early 1990s.

How PALs Work

To understand how PALs work, let’s break down their structure and programming process:

Structure of a PAL

A typical PAL consists of two main components:

- ***AND-plane*** : A programmable array of AND gates that receives inputs and creates product terms.
• OR-plane : A fixed array of OR gates that combines the product terms to create outputs.

The AND-plane is programmable, allowing designers to specify which inputs contribute to each product term. The OR-plane, being fixed, simply combines these product terms to produce the final outputs.

Programming Process

PALs are typically programmed using the following steps:

- ***Design*** : The logic function is designed using Boolean algebra or truth tables.
• Translation : The design is translated into a fusemap or a set of equations.
• Programming : The fusemap is burned into the PAL using a PAL programmer device.

Programming a PAL involves selectively “blowing” fuses in the AND-plane to create the desired connections. Once programmed, a PAL becomes a custom logic device tailored to the specific application.

Types of PALs

Several types of PALs have been developed to cater to different needs:

- ***Simple PALs*** : Basic devices with a programmable AND-plane and a fixed OR-plane (e.g., 16L8, 20L8).
• Registered PALs : Include flip-flops on the outputs for sequential logic (e.g., 16R4, 16R6, 16R8).
• Complex PALs : Offer more inputs, outputs, and product terms (e.g., 22V10).
• Generic Array Logic (GAL) : Erasable and reprogrammable version of PALs.

Each type offers different levels of complexity and functionality, allowing designers to choose the most appropriate device for their specific needs.

Applications of PALs

PALs have found applications in various fields of electronics, including:

- ***Address Decoding*** : In computer systems, PALs are often used to decode memory and I/O addresses.
• State Machines : Sequential logic circuits for controlling system behavior.
• Glue Logic : Interfacing between different components or bus systems.
• Protocol Conversion : Translating between different communication protocols.
• Embedded Systems : Implementing custom logic in microcontroller-based designs.
• Industrial Control : Creating specialized control circuits for machinery and processes.
• Consumer Electronics : Implementing custom functions in TVs, DVD players, and other devices.

The versatility of PALs makes them suitable for a wide range of applications where custom logic is required.

Advantages and Limitations of PALs

Like any technology, PALs come with their own set of advantages and limitations:

Advantages:

- ***Flexibility*** : Can be programmed to implement various logic functions.
• Reduced Time-to-Market : Faster to design and implement compared to custom ASICs.
• Cost-Effective : Cheaper for small to medium production runs.
• Simplified Inventory : One PAL can replace multiple discrete logic ICs.
• Improved Reliability : Fewer components and connections lead to higher reliability.

Limitations:

- ***Limited Complexity*** : Cannot implement very large or complex logic functions.
• One-Time Programmable : Most PALs can only be programmed once (except GALs).
• Speed : Generally slower than custom ASICs for the same function.
• Power Consumption : May consume more power than equivalent custom logic.

Programming PALs

Programming PALs involves several steps and tools:

- ***Design Entry*** : Logic functions are typically entered using schematic capture or hardware description languages (HDLs) like ABEL or PALASM.
• Synthesis : The design is synthesized into a form suitable for the target PAL device.
• Simulation : The design is simulated to verify correct operation before programming.
• Fuse Map Generation : A fuse map is created, specifying which fuses need to be blown.
• Device Programming : A PAL programmer device is used to physically program the PAL chip.

Modern PAL programming often uses software tools that integrate these steps, simplifying the process for designers.

Evolution: From PALs to CPLDs and FPGAs

While PALs revolutionized programmable logic, the demand for more complex and flexible devices led to further innovations:

Complex Programmable Logic Devices (CPLDs)

CPLDs can be seen as an evolution of PALs, offering more logic resources, reprogrammability, and often non-volatile configuration storage. They consist of multiple PAL-like blocks interconnected by a programmable switch matrix.

Key features of CPLDs:

- Higher logic capacity than PALs
• In-system programmability
• Faster speed compared to basic PALs
• Non-volatile configuration (retains programming when powered off)

Field-Programmable Gate Arrays (FPGAs)

FPGAs represent a further evolution, offering even greater flexibility and capacity:

- Very high logic capacity
• Reconfigurable in the field
• Often include specialized blocks (e.g., DSP blocks, memory blocks)
• Suitable for implementing entire systems-on-chip

While CPLDs and FPGAs have largely supplanted PALs in new designs, the principles behind PALs continue to influence modern programmable logic devices.

The Legacy of PALs

Although PALs are less common in new designs today, their impact on the field of electronics is undeniable:

- ***Democratization of Custom Logic*** : PALs made custom logic accessible to a wider range of engineers and small companies.
• Foundation for Modern PLDs : The concepts introduced by PALs laid the groundwork for more advanced devices like CPLDs and FPGAs.
• Education : PALs remain an excellent tool for teaching digital logic design principles.
• Ongoing Use : PALs are still used in certain applications, particularly in maintaining legacy systems.

Conclusion

Programmable Array Logic devices played a crucial role in the evolution of digital electronics, bridging the gap between inflexible discrete logic and expensive custom ASICs. Their ability to be customized after manufacture opened up new possibilities in circuit design and paved the way for more advanced programmable logic devices.

While PALs have largely been superseded by more complex devices like CPLDs and FPGAs in new designs, their legacy lives on. The principles behind PALs continue to influence modern programmable logic, and understanding PALs provides valuable insights into the foundations of digital circuit design.

As we continue to push the boundaries of electronic design, it’s worth remembering the impact of innovations like PALs. They remind us of the importance of flexibility, customization, and accessibility in driving technological progress. Whether you’re a seasoned engineer or a student of electronics, appreciating the role of PALs in the history of digital logic can provide valuable perspective on the field’s evolution and future directions.