Lattice GAL22V10B-25LP: Architecture, Key Features, and Application Design Considerations
The Lattice GAL22V10B-25LP stands as a classic and highly influential device in the realm of programmable logic. As a member of the Generic Array Logic (GAL) family, it provided a powerful, erasable, and pin-compatible successor to traditional PAL devices, offering designers unprecedented flexibility. This article delves into the architecture, defining features, and key considerations for designing with this iconic component.
Architecture: A Look Inside
The GAL22V10B-25LP is based on a well-established Programmable AND-Fixed OR array structure. Its architecture can be broken down into several key functional blocks:
1. Input/Output Logic Macrocells (OLMCs): The heart of its flexibility lies in its 10 output logic macrocells. Each macrocell can be individually configured by the user to operate as a dedicated input, a registered (clocked) output, or a combinatorial output. This configurability is managed through a programmable set of architecture control bits.
2. Programmable AND Array: This is the core programmable element. It consists of a large array of fusible links that allow users to define the product terms (AND operations) for each output function. The "22" in its name denotes the number of inputs to this array.
3. Fixed OR Array: Each output macrocell sums a fixed number of product terms from the AND array. The GAL22V10B provides a varying number of product terms per output (ranging from 8 to 16), a critical factor for designing complex logic functions.
4. Clock and Output Enable: The device features a global clock input for synchronous registered operations and a global output enable (OE) input, providing control over the tri-state outputs.
Key Features and Specifications
The "B" revision and the "-25LP" suffix specify its enhanced performance and package type.
High Performance: The -25 signifies a maximum propagation delay (`tPD`) of 25 ns, making it suitable for a wide range of medium-speed applications.
Low Power: The LP denotes Low Power, achieved through CMOS technology. This was a significant advantage over its bipolar predecessors, reducing overall system power consumption.
Electrically Erasable (EE) CMOS: Unlike one-time programmable (OTP) PALs, the GAL22V10B uses EECMOS technology, allowing it to be reprogrammed and tested numerous times. This drastically accelerated development cycles and reduced costs.
100% Testability: The architecture supports functional testing, ensuring high reliability.
10 Output Logic Macrocells: Provides significant logic density and I/O flexibility for its era.
Application Design Considerations
While designing with the GAL22V10B-25LP, several factors must be carefully considered to ensure a robust and functional implementation.
1. Product Term Allocation: The most critical design constraint is the limited and varying number of product terms per output. Complex logic functions requiring many AND terms must be mapped carefully across available outputs. Logic minimization techniques are essential.
2. Clock and Reset Management: The device has a single global clock. All registered outputs are synchronized to this same clock edge. Designs requiring multiple clock domains are not possible. Similarly, reset signals must be managed through the programmable array.
3. Pin-Locking: A key advantage is being pin-compatible with PALs. For drop-in replacements or stable PCB layouts, functions must be "pin-locked" early in the design process to avoid costly board re-spins.
4. Power-On Reset and State Machine Design: The power-on state of registered outputs is not guaranteed. Finite state machines (FSMs) must be designed with explicit reset states to initialize the system correctly upon power-up.
5. Modern Development Tools: While modern EDA tools may have limited direct support for older GAL devices, using them requires obtaining the original JEDEC fuse map files and programming them onto the device with a compatible programmer.
The GAL22V10B-25LP was a workhorse for implementing glue logic, state machines, address decoders, and complex interface logic in countless systems throughout the 1990s and early 2000s.
The Lattice GAL22V10B-25LP is a quintessential example of innovation in programmable logic. Its elegant macrocell-based architecture, CMOS low-power advantage, and reprogrammability solidified its role as a versatile and reliable solution for a generation of digital design, bridging the gap between simple PALs and more complex CPLDs.
Keywords:
Programmable Logic Device (PLD)
Output Logic Macrocell (OLMC)
Generic Array Logic (GAL)
Electrically Erasable CMOS (EECMOS)
Product Term
