Often it seems we need our microcontroller to be doing several things at once such as monitoring inputs while controlling changing outputs. A single controller cannot do this in the literal sense but it can operate so quickly that it appears to be doing several things simultaneously. Sometimes, though, this can get quite complicated if polling or busy-wait style code is used. For example, consider the operation of multiplexing several seven segment displays. Unlike a simple LED which merely requires us to set or clear a port bit, the multiplexed displays need constant attention. In the Reaction Timer Redux exercise the displays required 15 milliseconds per rendering; 5 milliseconds for each of the 3 displays. If we wanted to display the value for a few seconds we simply looped around that code. Although this worked well for that application, such is not always the case. Consider the case of an event counter. This is a device that counts things and displays the current value. It might be used on an assembly line to keep track of the number of items that have passed by, perhaps via the item triggering a mechanical, photo-electric, magnetic or other kind of switch. We would like this device to present a display of the current count at all times yet we also need to be constantly checking the item switch. How do we do both? In the case of the Reaction Timer we could add code within the display loop that checks the item switch but if the switch activates and deactivates in under 15 milliseconds we could miss it while the displays are being written to. Fortunately, there is a nice solution to this problem.