synthesis strategy

Synthesis Strategy is the set of constraints and directives that guide logic synthesis to optimize for area, speed, or power — determining how the synthesizer maps RTL code to a gate-level netlist using standard cells from the target library.

Synthesis Objectives

• Area optimization: Minimize gate count / chip area. Use smallest cells, share logic, reduce fanout.
• Timing optimization: Meet clock frequency. Use faster cells, restructure critical paths, maximize parallelism.
• Power optimization: Minimize switching activity. Use lower-drive cells, clock gating, multi-Vt assignments.
• Most synthesis is timing-driven: Area and power come second to timing closure.

SDC (Synopsys Design Constraints) — Core Inputs

create_clock -name CLK -period 1.0 [get_ports CLK]  # 1GHz clock
set_input_delay -clock CLK -max 0.3 [all_inputs]    # Input arrival time
set_output_delay -clock CLK -max 0.2 [all_outputs]  # Output required time
set_max_area 0                                       # Minimize area after timing met

Synthesis Effort Levels

• Compile: Default — balances quality vs. runtime.
• Compile -scan: Include scan insertion during synthesis.
• Compile_ultra: Maximum quality — structural optimizations, datapath restructuring. Slower.
• Incremental Compile: Fix specific paths without re-synthesizing entire design.

Path Groups

• Define priority groups: Critical path group gets highest optimization effort.
• Example: Clock_path > Reg2Reg > In2Reg > Reg2Out.
• Allocate timing budget to each path group.

Timing-Driven Logic Restructuring

• Register retiming: Move flip-flop boundary across combinational logic to balance path lengths.
• Logic duplication: Duplicate high-fanout cell to reduce net capacitance.
• Constant propagation: Propagate known constants (tied signals) through logic.
• Resource sharing removal: Sharing multipliers saves area but increases path depth.

Multi-Vt Assignment

• Start with SVT cells everywhere.
• Upsize to LVT on critical paths (timing closure).
• Downsize non-critical paths to HVT (leakage savings).
• Typical: 10–20% LVT, 60–70% SVT, 20–30% HVT for balanced design.

Synthesis strategy is the translator between designer intent and physical gates — the quality of synthesis determines whether the design can close timing, meet area targets, and achieve power budgets long before physical design begins.