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(https://reference.digilentinc.com/_detail/reference/programmable-logic/nexys-a7/n4s.png?id=reference%3Aprogrammable-logic%3Anexys-
a7%3Areference-manual) Figure 9.1.1 An Un-illuminated Seven-Segment Display and Nine Illumination Patterns Corresponding to Decimal Digits
The anodes of the seven LEDs forming each digit are tied together into one “common anode” circuit node, but the LED () cathodes remain
separate, as shown in Fig 18. The common anode signals are available as eight “digit enable” input signals to the 8-digit display. The
cathodes of similar segments on all four displays are connected into seven circuit nodes labeled CA through CG. For example, the eight “D”
cathodes from the eight digits are grouped together into a single circuit node called “CD.” These seven cathode signals are available as
inputs to the 8-digit display. This signal connection scheme creates a multiplexed display, where the cathode signals are common to all digits
but they can only illuminate the segments of the digit whose corresponding anode signal is asserted.
To illuminate a segment, the anode should be driven high while the cathode is driven low. However, since the Nexys A7 uses transistors to
drive enough current into the common anode point, the anode enables are inverted. Therefore, both the AN0..7 and the CA..G/DP signals
are driven low when active.
(https://reference.digilentinc.com/_detail/reference/programmable-logic/nexys-a7/n4t.png?id=reference%3Aprogrammable-logic%3Anexys-
a7%3Areference-manual) Figure 9.1.2 Common Anode Circuit Node
A scanning display controller circuit can be used to show an eight-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-eighth 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 eight 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 1/8 of the refresh cycle, or 2ms. 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 9.1.3.
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