Due to the rapid development of digital processor technology, various methodologies for design, development, implementation and testing of digital control systems have been proposed. In most cases, the systems are soft real-time or even non real-time, hardware-dependent or based on proprietary solutions. In this paper, we focus on a testing procedure for a microcontroller-based control, which will be adopted for software testing. The procedure is performed with no realtime constraints, but is capable to give information about the real-time conditions. It is called Processor in the Loop (PIL) and it is illustrated by the design and implementation of a DC motor speed control system; simulation and experimental results are then presented and analyzed.