Nexys 3™ FPGA Board Reference Manual
Copyright Digilent, Inc. All rights reserved.
Other product and company names mentioned may be trademarks of their respective owners.
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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
asserted, then a "7" will be displayed in digit
position 2. If AN0 and CB, CC are driven for
4ms, and then A1 and CA, CB, CC are driven
for 4ms in an endless succession, the display
will show "17" in the first two digits. An example timing diagram for a four-digit controller is provided.
10 Expansion Connectors
The Nexys 3 board has a 68-pin VHDC connector for high-speed/parallel I/O, and an 8-pin Pmod port for lower
speed and lower pin-count I/O.
10.1 VHDC Connector
The VHDC connector includes 40 data signals (routed as 20 impedance-controlled matched pairs), 20 grounds (one
per pair), and eight power signals. This connector, commonly used for SCSI-3 applications, can accommodate data
rates of several hundred megahertz on every pin. Both board-to-board and board-to-cable mating connectors are
available. Data sheets for the VHDC connector and for mating board and cable connectors can be found on the
Digilent website, as well as on other vendor and distributor websites. Mating connectors and cables of various
lengths are also available from Digilent and from distributors.
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
