Huffman Encoding — Hardware Implementation in Verilog

A hardware-based lossless text compression and decompression system implemented in Verilog, targeting the SkyWater 130nm (sky130) PDK. The design encodes and decodes English text using a predefined Huffman tree optimized for English character frequency distribution.

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Overview

Huffman Encoding assigns shorter binary codewords to more frequent characters and longer codewords to rarer ones, achieving efficient lossless compression. This project implements the full encode-decode pipeline as synthesizable RTL, verified through a file-driven Verilog testbench, and targeted for ASIC implementation using an open-source EDA flow.

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Architecture

The system is built around a single dual-mode RTL module controlled by the SEL signal.

Dual-Mode Module (decoder.v)

SEL	Mode	Input	Output
0	Encoder	8-bit ASCII char	16-bit Huffman code + length
1	Decoder	16-bit encoded bits	8-bit recovered ASCII char

Encoding Table Format

Each entry in the encoding file is a 28-bit word:

[27:24] — Code length (4 bits)
[23:8]  — Huffman code, left-justified (16 bits)
[7:0]   — ASCII character value (8 bits)

How It Works

Encoding (SEL=0)

1. Testbench reads one ASCII character at a time from textfile
2. Testbench scans the encoding file entry by entry, presenting each 28-bit entry to the DUT
3. DUT compares input_ascii against encoding[7:0]; on match, outputs the Huffman code and length
4. Testbench writes the Huffman bits to outputfile.txt
5. Process repeats until ETX (0x03) is encountered

Decoding (SEL=1)

1. Testbench reads the compressed bitstream from outputfile.txt, assembling 16-bit chunks
2. DUT accumulates bits in a 32-bit internal buffer
3. Testbench scans the encoding file; DUT compares the top bits of its buffer against each entry
4. On match, DUT outputs the recovered ASCII character and removes the consumed bits from the buffer
5. Testbench writes the recovered character to outputfile2.txt

EDA Tool Flow

Stage	Tool
Simulation	Icarus Verilog (iverilog)
Waveform Viewing	GTKWave
Synthesis	Yosys + sky130 PDK
Place and Route	OpenROAD
Physical Verification	Magic
PDK	SkyWater 130nm (sky130)

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