Lecture Overview

Course: CENG335: Digital Logic IILecturer: Dr. Hussein Kobeissi

Lecture Topics:

1. Programmable Logic Devices (PLDs) and FPGA

Importance:

• PLDs and FPGAs are crucial components in the design and implementation of complex digital logic circuits.

• Their inherent programmability allows designers to customize devices for specific applications, leading to enhanced flexibility and efficiency in circuit designs.

2. Programmable Logic Devices (PLDs)

Definition:

• PLDs are integrated circuits designed to be configured to perform various logic functions, providing a versatile solution in digital logic design.

3. Types of PLDs

3.1. Programmable Logic Arrays (PLA)

• Structure:

  • PLAs consist of a matrix of programmable AND gates followed by fixed OR gates. This structure facilitates complex logic implementations.

• Functionality:

  • Highly customizable since both the AND and OR planes can be programmed, enabling the realization of sophisticated logic functions tailored to specific needs.

3.2. Programmable Array Logic (PAL)

• Features:

  • PALs are characterized by programmable AND gates combined with fixed OR gates. This design aids in achieving faster operational speeds because the routing of OR gates is predetermined.

• Advantages:

  • Generally simpler in design and less expensive compared to PLAs; hence, PALs are often the go-to choice for applications requiring less complex logic configurations.

3.3. Complex Programmable Logic Devices (CPLD)

• Structure:

  • CPLDs consist of several programmable logic blocks capable of implementing extensive logic functions more efficiently on a single chip.

• Applications:

  • These devices are particularly suited for medium-scale applications where integration of several functions or gates is necessary, allowing for rich functionality in a compact form factor.

3.4. Field Programmable Gate Arrays (FPGA)

• Structure:

  • Comprised of a large array of logic blocks, input/output (I/O) blocks, and programmable interconnections; this architecture provides a highly configurable landscape for custom logic designs.

• Scalability:

  • FPGAs can accommodate large and complex circuits with millions of gates, making them ideal for advanced applications across different domains like telecommunications, digital signal processing, and more.

4. Implementation Examples with Fixed Chips

• Discussed practical examples involving fixed-function chips, such as the 7400 series, to illustrate the inherent limitations in customization when designing large circuits.

• Highlights the necessity for utilizing programmable devices in ensuring flexibility during circuit design and meeting diverse application requirements.

5. PLA Functionality and Configuration

• Configuration Process:

  • Inputs are routed to the AND plane to generate product terms, which are subsequently processed to yield outputs from the OR plane that represent specific sum functions.

• Example Functions in PLA:

  • f1 = x1 * x2 + x1' * x3 + x1' * x2 * x3

  • f2 = x1 * x2 + x1' * x2 * x3 + x1 * x3

6. Limitations of PLA Compared to PAL

• While PLAs offer full programmability in both AND and OR gates, this flexibility can result in slower operational speeds.

• In contrast, PALs are designed with programmable AND gates and fixed OR gates, optimizing for faster circuit design due to fewer programmable pathways.

7. CPLD Characteristics

• CPLDs feature multiple PAL-like blocks that work together, making them suitable for moderate-level circuit implementations.

• Size Example:

  • A typical CPLD may contain approximately 500 blocks, which can potentially implement around 10,000 gates, showcasing its capability in managing intricate designs and larger logic functions.

8. FPGA Structures and Capabilities

• Logic Block Usage:

  • FPGAs leverage logic blocks to form a configurable architecture capable of handling various computational tasks and adapting to numerous circuit requirements.

• Lookup Tables (LUTs):

  • LUTs are critical components within FPGAs, serving as small lookup functions based on the combinations of input signals.

  • Example:

    • A two-input LUT can implement any binary function of two variables, greatly simplifying complex logical operations and maximally utilizing space.

9. Programming and Circuit Implementation in FPGA

• Programming Process:

  • In constructing designs using FPGA technology, each function must be fit into assigned LUTs to assure compatibility and functionality within the overall circuit.

• Non-volatile Memories (PROM):

  • These serve to retain circuit configurations even when powered off, thereby enhancing reliability and persistence for various applications while reducing the risk of losing critical design settings.

10. Steps for Designing Circuits Using LUTs

• Emphasizes mapping of logic functions into LUTs correctly to optimize space and performance accordingly within FPGA configurations, enhancing the overall efficiency of the design procedures.

11. Comparison of Adder Designs

• Types of Adders:

  • Half-Adder:

    • Capable of adding two input bits and generating a sum alongside a carry output.

  • Full-Adder:

    • Advanced from half-adders, this configuration can add three bits, including a carry input, for more comprehensive arithmetic calculation.

  • Ripple-Carry Adder:

    • Integrates multiple full-adders, though it can experience delays due to the sequential nature of carry propagation across stages of addition.

12. Advanced Adder Types

• Carry-Lookahead Adder (CLA):

  • Innovates in design by minimizing delays through predictive calculations for carry outputs, greatly enhancing speed in arithmetic operations.

• Hierarchical Carry-Lookahead Adder:

  • Further improves performance by breaking down large designs into smaller, manageable blocks to simplify calculations and increase total processing speeds.

13. Performance and Trade-offs in Adder Designs

• Emphasizes that the design performance is markedly influenced by the delicate balance struck between speed and complexity associated with the logic circuit.

• Engineers often apply various strategies to ensure optimal performance while taking into account potential delays and the overall resource consumption of gates used within the design of the logical circuits.