Digilent 410-274P-KIT - Manual (Page 20)

Nexys4™ FPGA Board Reference Manual
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continuously illuminated. If the update, or “refresh”, rate is slowed to around 45 hertz, 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 1KHz 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 Fig 19.
AN0
AN1
AN2
AN3
Cathodes Digit 0
Refresh period = 1ms to 16ms
Digit period = Refresh / 4
Digit 1 Digit 2 Digit 3
9.2 Tri-Color LEDs
The Nexys4 board contains two tri-color LEDs. Each tri-color LED has three input signals that drive the cathodes of
three smaller internal LEDs: one red, one blue, and one green. Driving the signal corresponding to one of these
colors high will illuminate the internal LED. The input signals are driven by the FPGA through a transistor, which
inverts the signals. Therefore, to light up the tri-color LED, the corresponding signals need to be driven high. The
tri-color LED will emit a color dependent on the combination of internal LEDs that are currently being illuminated.
For example, if the red and blue signals are driven high, and green is driven low, the tri-color LED will emit a purple
color.
Note: Digilent strongly recommends the use of Pulse-Width Modulation (PWM) when driving the tri-color LEDs (for
information on PWM, see section 15.1). Driving any of the inputs to a steady logic ‘1’ will result in the LED being
illuminated at an uncomfortably bright level. You can avoid this by ensuring that none of the tri-color signals are
driven with more than a 50% duty cycle. Using PWM also greatly expands the potential color palette of the tri-
color led. Individually adjusting the duty cycle of each color between 50% and 0% causes the different colors to be
illuminated at different intensities, allowing virtually any color to be displayed.
10 Pmod Connectors
The Pmod connectors are arranged in a 2x6 right-angle, and are 100-mil female connectors that mate with
standard 2x6 pin headers.. Each 12-pin Pmod connector provides two 3.3V VCC signals (pins 6 and 12), two Ground
Figure 19. Four digit scanning display controller timing diagram