A Programmable Array Logic (PAL) device is a type of digital logic device that is used to implement combinational logic circuits. It is based on the concept of programmable AND array followed by a fixed OR array. The device allows users to program their own logic functions by defining the desired connections between inputs, outputs, and internal logic elements. PALs were widely used in the past for various applications, but they have largely been replaced by more advanced programmable logic devices such as Complex Programmable Logic Devices (CPLDs) and Field-Programmable Gate Arrays (FPGAs).
Here's how a typical PAL device functions:
Programmable AND Array: The PAL consists of a grid of product terms, where each product term is formed by combining input signals (both true and complemented) using AND gates. The inputs are connected to the AND gates, and the output of each AND gate is fed to the OR array.
Fixed OR Array: The output of each AND gate in the programmable AND array is connected to a fixed OR array. The OR array combines the outputs of the AND gates using OR gates to generate the final output signals.
Programming: To program the PAL, a user defines the desired logic function by specifying the connections between inputs, product terms (AND gates), and OR gates. This programming is typically done using a Hardware Description Language (HDL) or a specialized programming tool provided by the manufacturer.
Output Generation: Once the PAL is programmed, it behaves like a customized combinational logic circuit, implementing the logic function specified during programming.
Applications of Programmable Array Logic (PAL) devices include:
Glue Logic: PALs were widely used to implement glue logic in digital systems. Glue logic refers to the logic circuits needed to interface different components in a system that have incompatible logic levels or signal timing.
Prototyping: PALs were commonly used in the prototyping stage of digital system design. They allowed designers to quickly test and verify custom logic functions before moving to more complex and costly ASIC (Application-Specific Integrated Circuit) designs.
State Machines: PALs were used to implement state machines, such as sequencers and controllers, in various applications.
Peripheral Interface Logic: PALs were used to implement interface logic for connecting microprocessors or microcontrollers to external peripherals.
Simple Digital Circuits: PALs were used to implement various simple digital circuits, such as encoders, decoders, multiplexers, and demultiplexers.
While PAL devices were essential in the past for digital logic design, modern programmable logic devices like CPLDs and FPGAs offer more flexibility, higher logic capacity, and additional features, making them the preferred choice for most digital system designs today.