关于Synplify Pro两类SDC文件作用及SmartTime适配的疑问

Hey there! I’ve worked with Synplify Pro on countless RTL design flows, so let me break down exactly what each of these SDC files does, and how they fit into your synthesis, compilation, and timing analysis steps.

1. Your Manual my_design.sdc #

This is the user-defined "source" constraint file—it’s 100% under your control. You write it to define all core design constraints, like:

• Clock frequency, latency, and uncertainty
• Input/output (I/O) delays relative to external clocks
• False paths or multi-cycle paths that need special handling
• Any other custom timing rules specific to your design

This file is the foundation of your entire constraint strategy. It’s what tells the tool what you need the design to achieve, before any tool-specific processing happens.

2. Auto-Generated my_design_sdc.sdc #

When you run synthesis, Synplify Pro creates this file as a tool-processed derivative of your original SDC. Here’s what it does:

• It takes your manual constraints and converts them into a format optimized for the synthesis tool’s internal logic
• It adds auto-inferred constraints based on the synthesized netlist (e.g., adjusting clock relationships that emerge during optimization, or adding constraints on generated clock signals)
• It ensures consistency between your intent and how the synthesis tool actually interprets and applies those constraints

Think of it as a "translation" of your original constraints into something the synthesis engine can execute perfectly.

3. "For Synthesis" vs. "For Compilation" Settings #

When you right-click to assign these roles, you’re telling the tool which constraint file to use at each stage of the flow:

• Set my_design_sdc.sdc as "For Synthesis": This tells Synplify Pro to use the tool-optimized constraints during the synthesis run. Since this file is tailored to the synthesis engine, it ensures the tool applies your constraints accurately while optimizing the netlist for timing and area.
• Set it as "For Compilation": "Compilation" here refers to the downstream place-and-route (PnR) stage (e.g., in Xilinx Vivado or Intel Quartus). Using the derived SDC here ensures the PnR tool works with constraints that already account for synthesis-specific optimizations, avoiding mismatches between the synthesized netlist and the constraints used for layout.

4. SmartTime’s Use of These Files #

SmartTime (Synplify’s built-in timing analyzer) uses these files across different stages to validate timing consistency:

• Pre-synthesis: It loads your original my_design.sdc to do a "what-if" analysis, checking if your constraints are feasible before committing to synthesis.
• Post-synthesis: It uses my_design_sdc.sdc to perform timing analysis on the synthesized netlist, verifying that the optimized design meets your original timing goals.
• Post-PnR: If you link Synplify to your PnR tool, SmartTime will cross-reference the derived SDC with the PnR output to check for any timing drift introduced during layout, ensuring end-to-end timing convergence.

Quick Pro Tip #

Always make edits to your original my_design.sdc first, then re-run synthesis to generate a fresh my_design_sdc.sdc. This keeps your constraint source of truth clean and ensures all downstream stages use consistent, up-to-date constraints.

内容的提问来源于stack exchange，提问作者spaceman