Design Flow

• The design process must be understood in order to enhance design testability.
• Appending a DFT structure to the design at it completion may compromise the design goals.

Design Flow

• Design Synthesis
• Mapping one design description to another closer to the physical layer .
• Optimizing the design to suit the technology.

 

• CAD tools, however, go beyond synthesis, e.g.,
• Simulation
• TPG
• DRC
• Parameter extraction

 

• Synthesis is difficult because of design constraints and alternatives .

Design Flow

• Different tasks performed on the design.

• Here, Synthesis is defined as translation from Behavioral to Structural .
• RTL and Logic synthesis are examples.

 

• However, Generation is also synthesis (physical synthesis) of the masks:
• Synthesis based on std cell occupies the inner most layer of abstraction.
• Core-based synthesis would be at the outer most layer.

Design Flow

• Behavioral Synthesis shortens design cycle (and time-to-market) and allows design space to be explored.
• Power and testability added to traditional area and performance metrics.

 

• Behavioral synthesis (to RTL) consists of three main parts:
• Compilation
• Scheduling
• Allocations

 

• RTL optimization to improve area.

Design Flow

• Logic synthesis:
• Behavior synthesis produces an RTL module, that incorporates architectural elements such as registers, multiplexers and functional units.

 

• Some modules are part of a library, others need synthesized into logic.

 

• Two stages of logic synthesis
• Tech. independent
• Tech. dependent (technology mapping).

 

• The latter involves binding to library components, some of which are designed to facilitate testing, e.g.,
• Flip-flops with scan or for use in LFSR s for BIST .

Design Methodologies

• Two main categories:
• Full custom and semi-custom:

• Custom: maximizes design flexibility and performance .
• High development costs and time -- suitable only for large volume products such as memory and microprocessors.

 

• Semi-custom facilitated by having CAD tools use libraries of pre-designed components (and sometimes pre-fabricated chips: PLA and FPGAs).
• Automation restricts designers but enhances design time.

Design Methodologies

• Classification of methodologies:

• Trend is toward reuse of already designed and tested cells.
• Called cores or intellectual properties ( IP s).

 

• The classification of semi-custom design is based on the size and complexity of the library components.
• Cells are small gates, e.g., NANDs and NORs.
• Blocks are ALUs and multipliers.
• Cores are microprocessors and RAMs.

Design Methodologies

• Cores come in three types:
• Soft : A HDL description -- very flexible.
• Firm : mapped to a library.
• Hard : Layout description -- cannot be changed.

 

• Mask programmable types:

Type	AND plane	OR plane	Remark
PROM	Fixed	Program	E(E)PROM
PLA	Program	Program
PLD	Program	Fixed

 

• Unique failure modes are possible for the programmable devices, to be discussed.

 

• See text for other design details on MPGAs and FPGAs.

Physical Design

• Involves floor planning , P&R and parameter extraction .

• P&R algorithms discussed in the text.
• Placement needs to consider scan-path connectivity during synthesis.
• Routing needs to be consider signal integrity issues (e.g., crosstalk ) in addition to minimal distance and number of via heuristics.

 

• Back-annotation refers to extracting info from the circuit layout.
• Timing info feed back to synthesis stages for possible resynthesis.
• CAD tools used for parameter extraction called Technology CAD .