DigitalJS vs. RTL Bench: Technical & Architectural Comparison
DigitalJS pioneered rendering synthesized Yosys netlists directly as interactive browser canvases. While it remains an engaging educational tool for introductory logic experiments, real-world hardware engineering requires scalable schematic layouts, robust multi-file simulation, and silicon timing verification. Here is an authentic comparison of how both architectures operate.
1. Architectural Overview: How Each Platform Works
How DigitalJS Works
DigitalJS is a client-side, gate-level interactive circuit visualizer and interpreter. HDL code is first synthesized into a JSON netlist using a WebAssembly build of Yosys. DigitalJS then uses JointJS and a force-directed (spring-embedder) graph layout algorithm to place logic cells on a 2D SVG canvas.
Critically, DigitalJS implements its own event-driven JavaScript simulation engine in the browser tab. Users can directly click on primary inputs to toggle pins between 0 and 1, visually watching color changes propagate through gates in real time. Because the layout uses physics-based repulsion, gates float dynamically, and every bit is represented as an individual wire.
How RTL Bench Works
RTL Bench operates as a cloud-accelerated hardware elaboration workbench and physical toolchain. It uses Yosys and GHDL on server workers to synthesize Verilog, SystemVerilog, and VHDL into the RTLIL intermediate representation.
Instead of using spring-physics layouts, RTL Bench uses the Eclipse Layout Kernel (ELK) to produce
clean, layered orthogonal schematics: components are placed in left-to-right topological order, and all interconnects
route strictly with 90° bends. Multi-bit wires are collapsed into annotated buses (e.g. [31:0]).
Simulation is powered by native Icarus Verilog with full VCD waveform streaming, and netlists are analyzed
against open foundry Liberty libraries (SkyWater 130nm, GF180, IHP SG13G2) or compiled to FPGA bitstreams via nextpnr.
| Technical Dimension | DigitalJS | RTL Bench |
|---|---|---|
| Schematic Layout Algorithm | Force-directed layout Spring-embedder physics | ✓ Orthogonal layered routing Eclipse Layout Kernel (ELK) |
| Vector Bus Representation | Unbundled wires Draws discrete individual lines | ✓ Bus collapsing Annotated bit widths (e.g. [31:0]) |
| Source Code Cross-Probing | — No source linking | ✓ Bidirectional Click cell to jump to HDL source line |
| Simulation Execution Model | Interactive JS interpreter Click pins to toggle states live | Native Icarus Verilog Batch compiled testbench engine |
| Large RTL & Multi-File Scalability | Client-side browser tab Struggles with large pipelines | ✓ Cloud compilation Scales to multi-module designs |
| Waveform Viewer | Integrated canvas plot Basic signal lines | ✓ Full VCD drawer Dual cursors, zoom & derived Δt |
| PDK Liberty Delay Estimation | — Untimed functional simulation | ✓ Nominal cell lookup Sky130, GF180, IHP SG13G2 |
| VHDL & Mixed Language | Verilog only | ✓ Verilog, SV & VHDL-2008 Native GHDL integration |
| Physical FPGA Deployment | — | ✓ WebSerial flashing Tang Nano 9K/20K & iCE40 |
2. Honest Assessment: Merits & Demerits
Where DigitalJS Excels
- Hands-On Tactile Learning: Clicking an input pin on the canvas to see the output light bulb instantly turn yellow provides unbeatable intuitive feedback for beginners learning logic gates.
- Zero Backend Server Dependency: Because synthesis runs via WebAssembly and simulation runs in pure JavaScript, it can be deployed on static GitHub Pages with zero cloud hosting costs.
- Visual Clock Stepping: Lets students step clock cycles forward one tick at a time and see D flip-flops change color interactively.
Where DigitalJS Falls Short
- Wire Clutter on Non-Trivial RTL: Force-directed spring layouts cause diagonal wire overlaps. On multi-bit ALUs or state machines, the canvas quickly turns into an unreadable tangle.
- No Bus Collapsing: Expanding individual lines for every bit of a wide bus makes viewing data pipelines practically impossible.
- No Real-World Silicon Timing: Does not evaluate gate delays, setup/hold slack, or critical paths.
- No Hardware Flashing: Purely a software demonstration canvas with no FPGA synthesis flow.
Where RTL Bench Excels
- Production-Grade Schematics: ELK layered routing preserves standard left-to-right signal flow with clean 90° bends and collapsed buses, mirroring professional EDA digital schematic viewers like Cadence Genus or Vivado RTL Viewer.
- Source Cross-Probing: Clicking any cell in the schematic immediately highlights the exact line of HDL in the editor that generated it.
- Pre-Layout Timing Metrics: Evaluates register-to-register paths, critical gate depths, and setup slack against Liberty characterizations (SkyWater 130nm, GF180, IHP).
- Hardware Deployment: Synthesizes bitstreams via nextpnr and writes them directly to physical Tang Nano and iCE40 boards over WebSerial.
Where RTL Bench Falls Short
- No Interactive Canvas Pin Toggling: You cannot click an input pin on the schematic to toggle it interactively—inputs must be driven via testbenches or the automated stimulus generator.
- Cloud Execution Dependency: Requires a network connection to RTL Bench microservices rather than running completely offline in local browser memory.
3. The Verdict: Which Tool Should You Use?
Use DigitalJS if: You are an introductory computer engineering student or high school teacher exploring fundamental logic gates (AND, OR, XOR, half-adders) and want to physically click buttons on a screen to watch voltage levels toggle live.
Use RTL Bench if: You are writing synthesizable RTL (Verilog, SystemVerilog, or VHDL) and need a professional schematic view that scales to wide buses and multi-stage pipelines. It is the tool of choice when you need source-code cross-probing, full VCD waveforms with cursor measurements, rapid foundry delay estimation, or a direct path to programming physical FPGA hardware.
Frequently asked questions
Choose DigitalJS if you are an educator or student learning digital logic fundamentals who wants an interactive breadboard-style visualization with clickable input switches and real-time LED blinking.
Choose RTL Bench when working on real-world synthesizable RTL: you need clean, readable schematics, fast VCD waveform inspection, topological Liberty delay estimation, and direct FPGA bitstream deployment.
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