This paper presents a software and hardware validation of the attitude determination system for ST3LLARsat-1 BOIRA, the first student CubeSat developed at Universidad Carlos III de Madrid (UC3M). The study details the design of a dedicated photodiode configuration and a corresponding Sun-vector estimation algorithm. Moreover, an attitude determination framework based on the fusion of magnetometer, Sun vector, and gyroscope measurements was implemented using a quaternion-based extended Kalman filter (EKF), together with a method for monitoring filter convergence. Initial validation using model-in-the-loop simulation showed that, for the selected photodiode configuration, at least three photodiodes are illuminated for over 85% of the orbital time, keeping the Sun-vector estimation error below 5 deg. In addition, the EKF provided an RMS attitude estimation error of approximately 3 deg during eclipse, which represents the worstcase operating condition. The proposed convergence assessment method successfully identified unconverged filter behavior under intentionally disturbed conditions. Finally, the EKF was experimentally validated using an air-bearing free-rotation table to emulate a realistic operating scenario. By comparing the estimated attitude with the reference attitude obtained from an Attitude Heading and Reference System (AHRS), while using its internal sensor measurements as inputs to the developed EKF, the experimental results showed a maximum RMS attitude estimation error of 0.8 deg.