AMS designer flex mode gives you the flexibility to choose independent versions of Xcelium and Spectre simulation engines. It also provides you direct access to enhancements in the Spectre engine. Starting XCELIUM 20.03 s005, AMS designer flex mode is enabled by default if SPECTRE 19.1 ISR6 or later is found in the system path or is specified using the xrun -Spectre_path option.
From the XCELIUM 20.09 release, you need SPECTRE 19.1ISR6 or later release in your path when running AMS (Analog Mixed Signal) or RNM (Real Number Modeling) applications that include Spectre files (*.scs)
<SPECTRE_install_dir>/bin