Modeling space and design tools

• Axes and Levels of Representation

  
  Full detailed implementation occurs "at the center" of the Gajski-Kuhn diagram. Initial design specification begins at the outer boundaries of the diagram, especially at the behavioral axis with minimal, essential structural information (i.e., data structure) to be able to create a procedural description.

  VHDL does not support physical dimension very well -- but that is what synthesis must map the design to. Vendor supplied and user-specified attributes help to make VHDL adequate for mapping to the physical dimension of design.

• Information shown on each axis of representation

  
  VHDL:
  • is very good for behavioral description
  • involves verbose structural descriptions,
  • provides outstanding configuration management for structures,
  • does not support physical dimension very well -- but that is what synthesis must map the design to.

  It is extremely important to ensure that

  • the hierarchical form is consistent from axis to axis
  • information is consistent across the entire design (across all viewpoints of the design)

• Domains of activity for design tools

  
  Note that simulation occurs on the behavioral axis. Even structural models (e.g. netlists) have to be interpreted behaviorally by event protocols on the nets themselves, and by procedural models of the elements (e.g. gates and flipflops) being simulated.

The synthesis process requires

• Translation between axes. Substitution of an equivalent structural architecture for a behavioral module.

  Example: Replacing a procedural description of the operations to be performed by an ALU with the structure for the ALU.

• Translation down an axis. Substitution of a more detailed description for the higher level abstraction

  Example: substituting the gate level description for the MSI symbol of a multiplexor)

  Example: replacing integer range 0 to 255 with std_logic_vector ( 7 downto 0)

  It is critically important that as the design proceeds to increasing detail and from behavioral axis => structural axis => physical design that changes to the specification be reflected back into the more abstract models of the initial design.

  This is especially important in adding detail to the design: e.g. synchronization and timing, back annotation as drive capabilities and path delays are determined.

Alternative paths to realization exist:

• full custom design: structure to flexible cells
• standard cells: substitute structural models for equivalent cell layouts
• gate arrays: map gate models into FPGAs
• PLA's: map directly from behavioral equations and finite state machines to PLAs

Rev. 8/31/95 B. Huey