13.1 PLD Architectures and Programming

Programmable Logic Devices (PLDs) revolutionize digital design by offering flexible, customizable hardware solutions. From simple PALs to complex FPGAs, these devices enable designers to implement a wide range of digital circuits without the need for custom chip fabrication.

PLDs use various programming technologies, from one-time programmable fuses to reprogrammable flash memory. This flexibility allows for rapid prototyping, in-field updates, and design iterations, making PLDs a powerful tool for modern digital system development.

PLD Architectures

Architectures of programmable logic devices

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• Programmable Array Logic (PAL)
  • Fixed AND array combined with programmable OR array enables simple, one-time programmable structure
  • Limited complexity and flexibility constrains design possibilities (basic combinational and sequential logic)
• Complex Programmable Logic Device (CPLD)
  • Multiple PAL-like blocks interconnected by programmable switch matrix increases capacity and flexibility
  • Non-volatile configuration memory retains programming when powered off (EEPROM or Flash)
• Field-Programmable Gate Array (FPGA)
  • Array of configurable logic blocks (CLBs) connected by programmable interconnects offers high flexibility
  • Look-up tables (LUTs) implement combinational logic functions efficiently
  • Flip-flops enable sequential logic implementation
  • Embedded memory blocks and specialized functions enhance performance (DSP blocks, high-speed transceivers)

Programming technologies for PLDs

• Fuse-based programming
  • One-time programmable (OTP) technology permanently alters device configuration
  • Fuses blown to create desired connections form irreversible logic paths
  • Used in early PALs due to simplicity (22V10 devices)
• EPROM (Erasable Programmable Read-Only Memory)
  • Reprogrammable using UV light erasure allows multiple design iterations
  • Requires removal from circuit for reprogramming limits in-system flexibility
• EEPROM (Electrically Erasable Programmable Read-Only Memory)
  • Electrically erasable and reprogrammable in-circuit increases convenience
  • Slower write times compared to flash memory impacts configuration speed
• Flash-based programming
  • Fast electrical erasure and reprogramming enables quick design changes
  • Non-volatile storage maintains configuration without power
  • Used in modern CPLDs and some FPGAs due to speed and convenience (Xilinx Spartan-3AN)

PLD Implementation and Analysis

Logic circuit implementation with PLDs

• Combinational logic implementation
  • Boolean equations or truth tables define logic functions translated to PLD architecture
  • Synthesis tools optimize design for area and speed constraints
• Sequential logic implementation
  • State machine design using state diagrams or HDL captures complex behavior
  • Flip-flops and registers in PLD form memory elements
  • Clock domain management ensures proper timing across design
• Programming tools and languages
  • Hardware Description Languages (HDLs) describe circuit behavior (VHDL, Verilog)
  • Schematic capture tools provide graphical design entry
  • Synthesis and place-and-route software maps design to PLD resources
• Design verification
  • Simulation using testbenches validates functional correctness
  • Timing analysis ensures design meets speed requirements

PLDs vs fixed-function ICs

• Advantages of PLDs
  • Flexibility and reprogrammability enable rapid prototyping and design iterations
  • Reduced time-to-market accelerates product development
  • Lower inventory costs result from one device supporting multiple designs
  • In-field updates possible for some PLD types extend product lifecycle
• Limitations of PLDs
  • Higher unit cost for low-volume production impacts overall system cost
  • Increased power consumption compared to ASICs affects battery life in portable devices
  • Potentially lower performance than custom-designed ICs in specific applications
  • Learning curve for design tools and methodologies requires investment in training
• Comparison with fixed-function ICs
  • PLDs offer customization while fixed-function ICs provide pre-designed functionality
  • Fixed-function ICs typically have lower cost in high volumes due to economies of scale
  • PLDs allow for design changes fixed-function ICs cannot accommodate
  • Fixed-function ICs may have better performance in specific applications (standard logic families)