In recent years UAVs (unmanned aerial vehicles), also called drones for simplicity, are becoming more and more widespread all over the world. Born for military employment, now they are used for everyday tasks and also in the field of research. Here we want to show our implementation of a working firmware for UAV. The firmware main task is to link the software and the hardware. This is a key aspect because all sensors, motors and communication board must exchange a big quantity of data with each other. Moreover, those data are inputs or outputs of the software that is the 'brain' of the drones.