Programming Array Logic

Type of programmable logic device used to implement custom combinational logic circuits. It consists of a programmable AND gate array followed by a fixed OR gate array.

• PALs are easier to design and faster to use than Programmable Logic Arrays (PLAs), though they offer less flexibility.
• They are commonly used in digital systems to build specific logic functions efficiently with fewer components.

Key Components

Programmable AND Array

The AND array consists of multiple AND gates whose input connections can be programmed by the designer. This allows the creation of different product terms by combining input variables.

Fixed OR Array

The OR array consists of fixed OR gates that combine the outputs of the programmable AND array to produce final outputs.

Inputs and Outputs

PAL devices have multiple inputs that feed into the AND array and a fixed number of outputs generated by the OR array.

Programming Mechanism

PALs are programmed using hardware description languages (HDL) or specialized programming tools. The programming defines how inputs are connected in the AND array.

Comparison with other Programmable Logic Devices

Main difference between PLA, PAL and ROM lies in their structure:

• In PLA, programmable AND gate is followed by programmable OR gate.
• In PAL, programmable AND gate is followed by fixed OR gate.
• In ROM, fixed AND gate array is followed by programmable OR gate array.

Example: Realize the given function by using PAL: Any form from Sum of Products (SOP) or Product of Sums (POS) can be used for realization of a Boolean function.

There are three inputs A, B, C and three functions X, Y, Z. Using Sum of Products (SOP) terms to express the given function as follows:-

 X(A, B, C)=\sum(2, 3, 5, 7)  

Y(A,B,C)=\sum (0,1,5)

Z(A,B,C)=\sum (0,2,3,5)

The following truth table helps in understanding the function for the given inputs:

Place ‘1’ in the output columns for the corresponding minterms specified in each function.

Finding X, Y, Z: Look for high min terms (function value is equal to 1 in case of SOP) in each function output:

X = A'B + AC

Y = A'B' + B'C

Z = A'B + A'C + AB'C The AND array is programmed to generate the required product terms, while the fixed OR array combines them to produce the final outputs.

Advantages

• High Efficiency: PAL devices efficiently implement combinational logic using a programmable AND array and a fixed OR array, reducing unnecessary hardware usage.
• Low Production Cost: PALs are more cost-effective compared to Programmable Logic Arrays (PLA) due to their simpler architecture and fixed OR array.
• High Security: Once programmed, the internal connections of PAL are difficult to modify, providing better protection against unauthorized changes.

Disadvantages

• Limited Flexibility: Once programmed, the configuration of the AND array cannot be altered limiting the flexibility of design compared to the more advanced programmable devices like Field Programmable Gate Arrays (FPGA).
• Size Constraints: The PAL devices may have a limited number of logic gates and inputs which can restrict the complexity of the logic circuits that can be implemented.
• Propagation Delay: The fixed structure of the OR array may introduce propagation delays that can affect the performance of the high-speed applications.

Applications

• Digital Signal Processing: The PALs are used in the various digital signal processing applications where custom logic is required for the filtering and signal manipulation.
• Control Systems: They are employed in the control systems to implement specific logic functions such as the state machines and control logic for the automation.
• Embedded Systems: The PALs are commonly found in embedded systems for the implementing simple logic functions without the requiring extensive hardware resources.
• Communication Systems: Used in communication systems for the routing signals and implementing encoding/decoding functions.