Digilent 410-182P-KIT - Manual (Page 19)

Nexys3 Reference Manual
Doc: 502-182 page 19 of 22
Seven-Segment Display
The Nexys3 board contains a four-digit common anode seven-segment LED display. Each of the four
digits is composed of seven segments arranged in a “figure 8” pattern, with an LED embedded in
each segment. Segment LEDs can be individually illuminated, so any one of 128 patterns can be
displayed on a digit by illuminating certain LED segments and leaving the others dark. 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. The common anode signals are available as four “digit
enable” input signals to the 4-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 four “D” cathodes
from the four digits are grouped together into a single circuit node called “CD”). These seven cathode
signals are available as inputs to the 4-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.
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-quarter 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 45 hertz, most people will begin to see the display flicker.
In order 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 1KHz to 60Hz. For example, in a 60Hz
refresh scheme, the entire display
would be refreshed once every 16ms,
and each digit would be illuminated for
¼ of the refresh cycle, or 4ms. The
controller must drive the cathodes with
the correct pattern when the
corresponding anode signal is driven.
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
An un-illuminated seven-segment display, and nine
illumination patterns corresponding to decimal digits
AN1
AN2
AN3
AN4
Cathodes Digit 0
Refresh period = 1ms to 16ms
Digit period = Refresh / 4
Digit 1 Digit 2 Digit 3
A
F
E
D
C
B
G
Common anode
Individual cathodes
DP
AN3 AN2 AN1 AN0
CA CB CC CD CE CF CG DP
Four-digit Seven
Segment Display