SDF (Standard Delay Format) files are generated by Static Timing Analysis (STA) tools at different stages of the VLSI (Very Large Scale Integration) design flow.
There are several STA tools available in the market that can generate SDF files. Some of the popular STA tools are PrimeTime from Synopsys, Tempus from Cadence, and Encounter Timing System from Cadence.
SDF files are generated at two main stages of the VLSI design flow:
RTL (Register Transfer Level) to Netlist Conversion:
The first stage where SDF files are generated is during the RTL to Netlist conversion stage. In this stage, the RTL design is converted into a gate-level netlist that represents the design in terms of cells and interconnects. The STA tool analyzes the design and generates an SDF file that contains the timing information for the cells and interconnects in the design.
Post-Layout Stage:
The second stage where SDF files are generated is during the post-layout stage. In this stage, the physical design is complete, and the layout is ready for fabrication. The STA tool analyzes the physical design and generates an SDF file that contains the delay information for the cells and interconnects based on their physical placement on the chip.
The SDF file generated in the post-layout stage is used for back-annotation, which involves updating the RTL design with the timing information obtained from the physical design.
In summary, STA tools such as PrimeTime, Tempus, and Encounter Timing System can generate SDF files at different stages of the VLSI design flow, namely, RTL to netlist conversion and post-layout stage.
STA is a process that verifies the timing requirements of a digital circuit design by analyzing the delay of signals between different elements of the design. It helps designers to ensure that their circuits will work correctly and meet performance specifications.
In STA, designers start with a high-level description of the circuit known as RTL (Register Transfer Level) design. The RTL design describes the functionality of the circuit in terms of registers, logic gates, and interconnections.
During the physical design stage, the RTL design is transformed into a physical layout that includes details such as routing, placement of components, and the addition of buffer elements to address signal delays. These modifications can impact the timing of the circuit and may result in timing violations.
To address these timing violations, designers perform back annotation. Back annotation involves extracting timing information from the physical design layout, usually in the form of an SDF (Standard Delay Format) file, and feeding it back into the RTL design. This updated RTL design is then analyzed again by the STA tool to verify the timing and check for any timing violations.
The SDF file contains delay information for each element of the circuit, including cell delays, net delays, input/output delays, and clock delays. By incorporating this information back into the RTL design, designers can ensure that the timing analysis is based on an accurate representation of the physical layout of the circuit.
The back annotation process is iterative, meaning that it may need to be repeated several times until all timing violations are resolved. Once the timing requirements are met, the design can proceed to the next stage of the design flow.
In summary, back annotation is the process of updating the RTL design with timing information obtained from the physical design, to ensure that the final design meets the required timing specifications and is free of timing issues.