Digilent Nexys2 Manuale di riferimento - Pagina 6

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Nexys2 Reference Manual

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 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.

Figure 9: Nexys2 seven-segment displays

AN1

AN2

AN3

AN4

Cathodes

Figure 10: Seven-segment display timing diagram

Page 6/17

An un-illuminated seven-segment display, and nine

illumination patterns corresponding to decimal digits

Refresh period = 1ms to 16ms

Digit period = Refresh / 4

Digit 0

Digit 1

Digilent

www.digilentinc.com

Digit 2

Digit 3

Doc: 502-134