Digilent 410-274P-KIT - Manual (Page 19)

Nexys4™ FPGA Board Reference Manual
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Page 19 of 29
can be displayed on a digit by illuminating certain LED segments and leaving the others dark, as shown in Fig 17. Of
these 128 possible patterns, the ten corresponding to the decimal digits are the most useful.
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 (so, 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
Nexys4 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.
A
F
E
D
C
B
G
Common anode
Individual cathodes
DP
AN3 AN2 AN1 AN0
CA CB CC CD CE CF CG DP
Eight-digit Seven
Segment Display
AN7 AN6 AN5 AN4
CA CB CC CD CE CF CG DP
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
Figure 17. An un-illuminated seven-segment display, and nine illumination patterns corresponding to decimal digits
Figure 18. Common anode circuit node