Programming and Applications of the Lattice GAL22V10D-25LPN CPLD
The Lattice GAL22V10D-25LPN stands as a classic and highly influential device in the realm of digital logic design. As a Complex Programmable Logic Device (CPLD), it serves as a versatile bridge between discrete logic gates and larger, more complex FPGAs. Its enduring popularity, even in an era dominated by advanced FPGAs and ASICs, is a testament to its simplicity, reliability, and effectiveness for a wide range of glue logic applications.
Architectural Overview
The GAL22V10D-25LPN is built upon a PAL (Programmable Array Logic)-like architecture. Its identifier provides key specifications: "22" indicates up to 22 inputs, "10" signifies 10 output logic macrocells, and "25" denotes a maximum propagation delay of 25 nanoseconds. The "-LPN" suffix refers to its low-power operation and its plastic leaded chip carrier (PLCC) package.
The core structure consists of a programmable AND array followed by a fixed OR array. The programmable AND array allows users to define product terms, which are then summed by the OR array to create specific sum-of-products logic functions. Each of the ten output macrocells is incredibly flexible, allowing designers to configure individual pins as combinatorial outputs, registered outputs, or dedicated inputs. This macrocells' programmability includes control over output polarity and register configuration, making the device adaptable to numerous logic requirements.
Programming the Device
Programming the GAL22V10D-25LPN is a straightforward process that has been standardized over decades. The primary toolchain involves:
1. Design Entry: The desired logic function is captured using Hardware Description Languages (HDLs) like VHDL or Verilog, or more traditionally, schematic entry or Boolean equations.
2. Logic Synthesis: The design is synthesized into a netlist representing the logic gates and their interconnections.
3. Fitting/Placement and Routing: The software tool (e.g., Lattice's ispLEVER Classic or third-party tools like WinCupl) maps the netlist onto the actual resources of the CPLD. It assigns the logic to the programmable AND array and configures the macrocells.
4. JEDEC File Generation: The output of the fitting process is a JEDEC file (JED), a standard format that contains the fuse map data for the device.
5. Device Programming: The JEDEC file is transferred to a dedicated CPLD programmer, which applies the necessary voltages and signals to electrically configure the device's internal fuses. The GAL22V10D-25LPN is a One-Time Programmable (OTP) device, meaning its configuration is permanently set.
Key Applications
The GAL22V10D-25LPN excels in applications that require "glue logic"—the interstitial logic that connects larger integrated circuits. Its key applications include:
Address Decoding: Generating chip select signals for memories and peripherals in microprocessor-based systems.
Bus Interface Logic: Acting as a buffer, translator, or controller for data buses (e.g., between a CPU and various I/O devices).
State Machine Implementation: Perfect for implementing medium-complexity finite state machines (FSMs) that control system sequences.
I/O Expansion and Pinout Remapping: Consolidating multiple discrete logic chips into a single CPLD, simplifying board layout and reducing component count.
Clock Division and Simple Signal Conditioning: Performing functions like frequency division, synchronization, and pulse generation.
Advantages and Considerations
The primary advantages of this CPLD are its ease of use, low non-recurring engineering (NRE) costs, and deterministic timing. Unlike FPGAs that load configuration from an external memory at power-up, a CPLD is instantly active, making it ideal for critical boot-up and control functions. Its simple, predictable structure makes timing analysis straightforward.
However, its OTP nature means designs cannot be updated in-circuit, and its logic density is minuscule compared to modern FPGAs. It is best suited for fixed-function, low-to-medium complexity logic replacements.
In summary, the Lattice GAL22V10D-25LPN CPLD remains a fundamental component for digital designers. It provides a robust, cost-effective, and power-efficient solution for integrating glue logic, implementing control functions, and simplifying PCB design. Its enduring legacy is a powerful example of how a well-architected, simple device can maintain relevance across generations of electronic systems.
Keywords: CPLD, Programmable Logic, Glue Logic, JEDEC File, Macrocells
