As a brief introduction, a stimulus is the external signal used to stimulate a device through I/O ports and registers. In other words, a stimulus is something from outside that might affect the behavior of a device. The word “asynchronous” means that the user can cause this stimulus to happen at any time during the simulation. The stimulus window is where all work in MPLAB will be done and stimuli will be defined. The workbook is where stimulus settings can be saved for future use.