Pacemakers are cardiac stimulation devices that provide minimal energy to the heart by applying a pacing current between two electrodes. We are interested in building a computational model of this pacing phenomena, that would output reliable estimates of the minimal energy to trigger an action potential. The ultimate goal is to carry out in-silico pre-clinical trials. I will summarize the path we choose to reach this objective and what is our current status. I will present a 3D model that couples the bidomain equations to a pacemaker, and a 0D surrogate model, results from the analysis of the 3D model, numerical results obtained with 0D simulations, and preliminary 3D ones. In addition, I will show how to establish the model's credibility by going through a verification and validation process. In particular, I'll explain how we calibrate the model against experimental results designed specifically to this aim, and plan to validate it. This work is part of the Simcardiotest European project, realized in collaboration with the company MICROPORT CRM.