The decoder module should convert a 4-bit binary number input into the corresponding segment control signals (CA, CB, ..., CG) required to illuminate the appropriate segments on a 7-segment display. For binary values ranging from 0000 to 1001, the decoder should generate the correct segment patterns. However, for any other binary input value outside of this range, the decoder should output a pattern that results in all segments remaining unilluminated (dark display). Additionally, the decoder should include an independent decimal point input signal (dp_in) to control the state of the decimal point (DP) segment separately.

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. 

To illuminate a segment, the anode should be driven high while the cathode is driven low.  

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The common anode signals are available as eight “digit enable” input signals to the 8-digit display. 

The Nexys4 DDR 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.

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