NI Digital System Development Board User Manual | © National Instruments | 41
Figure 17. Common Anode Circuit Node
A scanning display controller circuit can be used to show a four-digit number on this display.
This circuit drives the anode signals and corresponding cathode patterns of each digit in a
repeating, continuous succession at an update rate that is faster than the human eye can detect.
Each digit is illuminated just one-fourth of the time, but because the eye cannot perceive the
darkening of a digit before it is illuminated again, the digit appears continuously illuminated. If
the update, or refresh, rate is slowed to around 45Hz, a flicker can be noticed in the display.
For each of the four digits to appear bright and continuously illuminated, all four digits should
be driven once every 1 to 16ms, for a refresh frequency of about 1 kHz to 60Hz. For example,
in a 62.5Hz refresh scheme, the entire display would be refreshed once every 16ms, and each
digit would be illuminated for 1/4 of the refresh cycle, or 4ms. The controller must drive low the
cathodes with the correct pattern when the corresponding anode signal is driven high. To
illustrate the process, if AN0 is asserted while CB and CC are asserted, then a
1 will be displayed
in digit position 1. Then, if AN1 is asserted while CA, CB, and CC are asserted, a
7 will be
displayed in digit position 2. If AN0, CB, and CC are driven for 4ms, and then AN1, CA, CB,
and CC are driven for 4ms in an endless succession, the display will show
71 in the first two
digits. An example timing diagram for a four-digit controller is shown in Figure 18.
Figure 18. Four Digit Scanning Display Controller Timing Diagram
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