Implement Decoder in Verilog (gate level)

Objective

A Decoder is a combinational circuit that converts n input lines into 2ⁿ unique outputs.
It activates exactly one output line corresponding to the binary code of the input.
In this tutorial, we will design a 2-to-4 decoder in Verilog and verify it using a testbench.

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Explanation (How it works)

A 2-to-4 decoder has:

• Inputs: A1, A0 (2-bit binary input)
• Outputs: D0, D1, D2, D3

Decoding rules:

• If input = 00 → D0 = 1
• If input = 01 → D1 = 1
• If input = 10 → D2 = 1
• If input = 11 → D3 = 1

All other outputs remain 0.

Boolean equations:

• D0 = ~A1 & ~A0
• D1 = ~A1 & A0
• D2 = A1 & ~A0
• D3 = A1 & A0

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Requirements

1. Create a Verilog module named decoder2to4 with:
   • Inputs: A1, A0
   • Outputs: D0, D1, D2, D3
2. Implement the Boolean equations for each output.
3. Write a testbench module:
   • Apply all 4 input combinations (00, 01, 10, 11).
   • Verify that exactly one output is active at a time.

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Expected Truth Table

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Test Plan (What to do)

1. Simulate testbench and apply all input combinations.
2. Confirm only one output is high for each input code.
3. Compare simulation results with the truth table.

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Common Pitfalls

• Forgetting to invert the inputs for equations of D0 and D1.
• Activating multiple outputs at once (incorrect).
• Mixing up decoder vs encoder (decoder expands inputs, encoder compresses inputs).

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Extension (Challenge)

• Design a 3-to-8 decoder using the same method.
• Add an enable input (EN) that controls whether outputs are active or forced to 0.
• Implement a hierarchical decoder (e.g., build a 3-to-8 decoder using two 2-to-4 decoders).