What is time multiplexing?

Let's return to the design we used for a single digit display in lab 4. In that system, we used one MicroStamp11 output line to drive each LED segment. To display a particular number, we needed to set seven output lines at the appropriate level. If we now try to extend this approach to a dual digit display, then we would need to use 14 output lines to drive the display. Since the MicroStamp11 only has 11 output lines, we have a problem.

The usual solution is to build a multiplexed display. In a multiplexed display we connect all like segments of the digits together. In this way, no matter how many digits we have in our display, we only need the seven wires. To decide which display to turn on, we need a switch that connects the common anode terminal of the LED display to $+5$ volts. We would therefore need to have an additional wire for each digit of our display. A simple schematic diagram illustrating the ideas behind a multiplexed display is found in figure 1.

Figure 1: Multiplexed Display

The proper operation of a multiplexed display depends upon a feature of human visual perception known as flicker fusion. If a light is flashed quickly enough, individual flashes become imperceptible and the illusion of a steady light is created. This is the basis for movies, television, and fluorescent lighting. All of these devices flash quickly enough to give the illusion of steady light or movement. How fast must a light flash before flicker fusion occurs? As a general rule of thumb, any light flashing faster than 50 times per second will appear to be steady. But you will verify this number as one of the in-lab tasks.

Multiplexed displays only show one digit at a time, but by cycling through all digits repetitively and cycling very fast, a multi-digit display is perceived. To drive a particular digit, its common anode is connected to the appropriate supply voltage and the segments are driven as needed for the desired number in that place. The cycling through the digits must be done quickly enough for flicker fusion to occur and this is accomplished through the system's software.