Logic Synthesis

What is Logic Synthesis ?

“ Logic synthesis is the process of converting a high-level description of the design into an optimized gate-level  representation, given a standard cell library and certain design constraints “

Why Perform Logic Synthesis ?

1. Automatically manages many details of the design process:

• Fewer bugs

• Improves productivity 

2.  Abstracts the design data (HDL description) from any particular implementation technology

• Designs can be re-synthesized targeting different chip technologies;

E.g.: first implement in FPGA then later in ASIC


3. In some cases, leads to a more optimal design than could be

achieved by manual means (e.g.: logic optimization) 

Logic Synthesis Flow : RTL TO GATES

RTL description:

Design at a high level using RTL constructs.


Translation:

Synthesis Tool convert the RTL description to un-optimized Internal representation.(Boolean form)


Un-optimized Intermediate Representation:

Represented internally by the logic synthesis tool in terms of Internal data structure.

Logic Optimization: 

Logic is optimized to remove redundant logic. 

Technology Mapping and Optimization: 

The synthesis tool takes the internal representation and implements the representation in gates, using the cells provided in the technology library.

Technology Library: 

library cells that can be basic gates or macro cells.


The cell description contains information about the following:


• Functionality of the cell.

• Area of the cell layout.

• Timing information about the cell.

• Power information about the cell.

Design Constraints:

1.Area: 

• Designer can specify area constraint and synthesis tool will optimize for minimum area. 
• Area can be optimized by having lesser number of cells and by replacing multiple cells with single cell that includes both functionality. 

2. Timing: 

• Designer specifies maximum delay between primary input and primary output.

There are three types of critical paths:
I.Path between a primary input and primary output.
II.Path from any primary input to a register.
III.Path from a register to a primary output.
IV.Path from a register to another register


3. Power:

• Development of hand-held devices has led to reduction of battery size and hence low power consuming systems..

Points to note about synthesis 

• For very big circuits, vendor technology libraries may yield non- optimal result. 
• Translation, logic optimization and technology mapping are done internally in the logic synthesis tool and are not visible to the designer. 
• Timing analyzer built into synthesis tools will have to account for interconnect delays in the total delay calculation