Internal Error: Sub-system: CDB, File: /quartus/db/cdb/cdb_net_sys.cpp, Line: 3973 - Internal Error: Sub-system: CDB, File: /quartus/db/cdb/cdb_net_sys.cpp, Line: 3973 Description Due to a problem in the Quartus® II software version 12.0 SP1 and earlier, you may see this error if your design contains a tristate output pin if the Fitter packs the output enable register into an IO element and the data input of the tristate buffer is tied to a constant logic '1'. Resolution To avoid this problem, perform one of the following actions: Use the Fast Output Enable Register assignment set to Off to prevent the Fitter from packing the output enable register into the IO element Connect data input of the tristate buffer to a non-static signal This problem is fixed beginning with the Quartus II software version 12.1. Custom Fields values: ['novalue'] Troubleshooting novalue False ['novalue'] ['FPGA Dev Tools Quartus II Software'] 12.1 novalue ['Programmable Logic Devices'] ['novalue'] ['novalue'] ['novalue']