This paper proposes a method for microcontroller's IC-EMC model extension. The internal impedance can be estimated without any prior knowledge of microcontroller. The internal impedance is represented by “impulse response” (IR) function and built into an extended IC-EMC model. Theoretical deduction, simulation, and a case study are presented in this paper. Currently, the case study shows that the proposed method is accurate within limited bandwidth (below 30 MHz) due to measurement limitations and is expected to be more accurate with more advanced equipments. This method can help IC design companies to keep their intellectual properties (IP) inside while providing an IP-unrelated EMC model for IC-EMC modellers. It can also help IC-EMC modellers to determine different IC's internal impedance by themselves.