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Parallax Static Timing Analyzer

OpenSTA is a gate level static timing verifier. As a stand-alone executable it can be used to verify the timing of a design using standard file formats.

  • Verilog netlist
  • Liberty library
  • SDC timing constraints
  • SDF delay annotation
  • SPEF parasitics

OpenSTA uses a TCL command interpreter to read the design, specify timing constraints and print timing reports.

  • Generated
  • Latency
  • Source latency (insertion delay)
  • Uncertainty
  • Propagated/Ideal
  • Gated clock checks
  • Multiple frequency clocks
Exception paths
  • False path
  • Multicycle path
  • Min/Max path delay
  • Exception points
  • -from clock/pin/instance -through pin/net -to clock/pin/instance
  • Edge specific exception points
  • -rise_from/-fall_from, -rise_through/-fall_through, -rise_to/-fall_to
Delay calculation
  • Integrated Dartu/Menezes/Pileggi RC effective capacitance algorithm
  • External delay calculator API
  • Report timing checks -from, -through, -to, multiple paths to endpoint
  • Report delay calculation
  • Check timing setup
Timing Engine

OpenSTA is architected to be easily bolted on to other tools as a timing engine. By using a network adapter, OpenSTA can access the host netlist data structures without duplicating them.

  • Query based incremental update of delays, arrival and required times
  • Simulator to propagate constants from constraints and netlist tie high/low

See doc/OpenSTA.pdf for command documentation. See doc/StaApi.txt for timing engine API documentation. See doc/ChangeLog.txt for changes to commands.

OpenSTA is dual licensed. It is released under GPL v3 as OpenSTA and is also licensed for commerical applications by Parallax Software without the GPL's requirements.

OpenSTA is open source, meaning the sources are published and can be compiled locally. Derivative works are supported as long as they adhere to the GPL license requirements. However, OpenSTA is not supported by a public community of developers as many other open source projects are. The copyright and develpment are exclusive to Parallax Software. Contributors must signing the Contributor License Agreement (doc/CLA.txt) when submitting pull requests.

Removing copyright and license notices from OpenSTA sources (or any other open source project for that matter) is illegal. This should be obvious, but the author of OpenSTA has discovered two different cases where the copyright and license were removed from source files that were copied.

The official git repository is located at Any forks from this code base have not passed extensive regression testing which is not publicly available.


OpenSTA is built with CMake.


The build dependency versions are show below. Other versions may work, but these are the versions used for development.

         from   Ubuntu   Macos
                22.04.2  14.4.1
cmake    3.10.2 3.24.2   3.29.2
clang    9.1.0           15.0.0
gcc      3.3.2   11.4.0  
tcl      8.4     8.6     8.6.6
swig     1.3.28  4.1.0   4.2.1
bison    1.35    3.8.2   3.8.2
flex     2.5.4   2.6.4   2.6.4

Note that flex versions before 2.6.4 contain 'register' declarations that are illegal in c++17.

External library dependencies:

         from   Ubuntu   Macos
eigen           3.4 .0   3.4.0 required
tclreadline              2.3.8 optional
libz     1.1.4   1.2.5   1.2.8 optional
cudd             2.4.1   3.0.0 optional

The TCL readline library links the GNU readline library to the TCL interpreter for command line editing On OSX, Homebrew does not support tclreadline, but the macports system does (see To enable TCL readline support use the following Cmake option:


The Zlib library is an optional. If CMake finds libz, OpenSTA can read Verilog, SDF, SPF, and SPEF files compressed with gzip.

CUDD is a binary decision diageram (BDD) package that is used to improve conditional timing arc handling. OpenSTA does not require it to be installed. It is available here or here.

Note that the file hierarchy of the CUDD installation changed with version 3.0. Some changes to CMakeLists.txt are required to support older versions.

Use the USE_CUDD option to look for the cudd library. Use the CUDD_DIR option to set the install directory if it is not in one of the normal install directories.

When building CUDD you may use the --prefix option to configure to install in a location other than the default (/usr/local/lib).

cd $HOME/cudd-3.0.0
mkdir $HOME/cudd
./configure --prefix $HOME/cudd
make install

cd <opensta>/build
cmake .. -DUSE_CUDD=ON -DCUDD_DIR=$HOME/cudd

Installing with CMake

Use the following commands to checkout the git repository and build the OpenSTA library and excutable.

git clone
cd OpenSTA
mkdir build
cd build
cmake ..

The default build type is release to compile optimized code. The resulting executable is in app/sta. The library without a main() procedure is app/libSTA.a.

Optional CMake variables passed as -D= arguments to CMake are show below.

CMAKE_CXX_FLAGS - additional compiler flags
TCL_LIBRARY - path to tcl library
TCL_HEADER - path to tcl.h
CUDD - path to cudd installation
ZLIB_ROOT - path to zlib

If TCL_LIBRARY is specified the CMake script will attempt to locate the header from the library path.

The default install directory is /usr/local. To install in a different directory with CMake use:

cmake .. -DCMAKE_INSTALL_PREFIX=<prefix_path>

If you make changes to CMakeLists.txt you may need to clean out existing CMake cached variable values by deleting all of the files in the build directory.

Bug Reports

Use the Issues tab on the github repository to report bugs.

Each issue/bug should be a separate issue. The subject of the issue should be a short description of the problem. Attach a test case to reproduce the issue as described below. Issues without test cases are unlikely to get a response.

The files in the test case should be collected into a directory named YYYYMMDD where YYYY is the year, MM is the month, and DD is the day (this format allows "ls" to report them in chronological order). The contents of the directory should be collected into a compressed tarfile named YYYYMMDD.tgz.

The test case should have a tcl command file recreates the issue named run.tcl. If there are more than one command file using the same data files, there should be separate command files, run1.tcl, run2.tcl etc. The bug report can refer to these command files by name.

Command files should not have absolute filenames like "/home/cho/OpenSTA_Request/write_path_spice/dump_spice" in them. These obviously are not portable. Use filenames relative to the test case directory.


  • James Cherry

  • William Scott authored the arnoldi delay calculator at Blaze, Inc which was subsequently licensed to Nefelus, Inc that has graciously contributed it to OpenSTA.


OpenSTA, Static Timing Analyzer Copyright (c) 2023, Parallax Software, Inc.

This program is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.

This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.

You should have received a copy of the GNU General Public License along with this program. If not, see