Methods and circuits are disclosed for testing a partial circuit design including circuit modules having a set of ports configured to be driven by signals from ports of one or more circuits omitted from the partial circuit. The set of ports are identified by identifying ports that are not connected by a net to another port or input/output (I/O) pin in the circuit design and that form inputs to slave circuits in the circuit modules. A traffic generator circuit is added to the partial design to form a test circuit design. The traffic generator circuit is configured to provide to the set of ports respective input data signals having a pattern consistent with master-to-slave communication. Operation of a test circuit design is modeled. A set of data signals generated by the circuit modules during the modeled operation of the test circuit design is captured and stored.