Lattice GAL16V8D-25QPN: Architecture, Key Specifications, and Application Circuit Design
The Lattice GAL16V8D-25QPN stands as a classic and highly influential device in the history of programmable logic. As a member of the Generic Array Logic (GAL) family, it provided a powerful, erasable, and electrically reprogrammable alternative to fixed PAL devices, offering designers unparalleled flexibility in digital logic design.
Architecture Overview
The architecture of the GAL16V8D is ingeniously designed around a programmable AND array feeding into a fixed OR array, a structure known as a Programmable Logic Device (PLD). The "16" in its name denotes the number of inputs, while the "8" refers to the maximum number of outputs. Its core architectural components include:
Programmable AND Array: This is the heart of the device. It generates product terms (combinations of input signals and their complements) based on the user's programming. The array's configurability defines the device's logic function.
Output Logic Macro Cells (OLMCs): This is the key feature that differentiated GAL devices from their predecessors. Each of the eight outputs is fed by a sophisticated OLMC. The macrocell can be configured by the user to operate as a dedicated input, a dedicated combinatorial output, a registered output (with a D-type flip-flop), or a bidirectional I/O pin. This flexibility allows a single GAL device to implement a wide range of logic functions, from simple glue logic to complex state machines.
I/O Pins and Feedback: Outputs can be fed back into the AND array, enabling the creation of complex sequential circuits like counters and shift registers.
Key Specifications (GAL16V8D-25QPN)
The part number itself encodes critical specifications:
GAL16V8D: The device family and model.
-25: Signifies a maximum propagation delay (tPD) of 25 ns, making it suitable for clock speeds up to approximately 40 MHz.
QPN: Indicates the package type—a Plastic Leaded Chip Carrier (PLCC) with 20 pins.
Other essential specifications include:
Technology: CMOS for low power consumption.
Operating Voltage: +5V ±10% standard.
Programmability: Electrically Erasable (EE) CMOS technology, allowing for over 100 erase/write cycles.
Package: 20-pin PLCC, a common surface-mount package for its era.
Application Circuit Design
Designing with the GAL16V8D-25QPN typically follows a structured workflow:
1. Logic Definition: The desired logic function (e.g., address decoder, state machine, bus controller) is defined using truth tables, state diagrams, or Boolean equations.
2. Design Entry: The logic is entered using a Hardware Description Language (HDL) like VHDL or Verilog, or more traditionally, schematic capture or Boolean equations in a PLD development tool.
3. Compilation and Fitting: The software compiler (e.g., using a JEDEC file format) translates the design into a fuse map that configures the internal AND array and OLMCs.
4. Programming: The compiled JEDEC file is transferred to a dedicated programmer, which applies specific voltages to the chip to "burn" the configuration into its EECMOS cells.
5. Circuit Integration: The programmed GAL is then inserted into its target circuit board. A typical application circuit involves:
Connecting VCC (+5V) and GND.
Connecting the input signals from other ICs (e.g., a microprocessor's address bus) to the GAL's input pins.
Connecting the output pins to their destinations (e.g., chip select lines for memory chips).
For registered configurations, providing a system clock signal to the dedicated clock pin (Pin 1).
A common design example is a memory address decoder. A microprocessor may have a 16-bit address bus, but only need to select one of eight memory-mapped devices. The GAL16V8D can be programmed to decode specific address ranges (e.g., A15-A12) and generate a unique, active-low chip select (CS) signal for each device, significantly reducing the need for numerous discrete logic gates.
ICGOOODFIND
The Lattice GAL16V8D-25QPN is a cornerstone of programmable logic, renowned for its architectural flexibility through Output Macro Cells, solid 25ns speed performance, and CMOS low-power operation. It remains a quintessential solution for integrating "glue logic," implementing state machines, and creating custom decoders, embodying the critical transition towards highly configurable digital design.
Keywords:
1. Programmable Logic Device (PLD)
2. Output Logic Macro Cell (OLMC)
3. Programmable AND Array
4. Propagation Delay (tPD)
5. JEDEC File
