The synthesis process is not a single step but a series of intricate transformations and optimizations performed by sophisticated EDA tools.

This initial phase focuses on understanding the designer's intent from the HDL source code.

• Parsing and Lexical/Syntax Analysis: The synthesis tool first reads the HDL files, tokenizes the code, and performs a thorough check for syntax errors and correct language constructs.
• Semantic Analysis: The tool then interprets the meaning of the HDL code. It understands the types of signals and variables, how different modules are instantiated and connected, and resolves any hierarchical references.
• Internal Representation (Abstract Syntax Tree - AST): The tool builds an internal, technology-independent data structure representing the design's functionality. This is often an Abstract Syntax Tree (AST) or a control-dataflow graph. At this stage, the design is still abstract; there are no specific gates or LUTs yet. The tool comprehends what the circuit should do, not how it will be physically implemented. It resolves parameters, generates unrolled loops, and expands generate statements.

Once the internal representation is built, the synthesis tool applies various optimization techniques that are independent of the target hardware technology. The goal here is to simplify the logic, reduce redundancy, and prepare the design for efficient mapping.

• Boolean Equation Simplification: Applying Boolean algebra theorems (e.g., De Morgan's laws, consensus theorem, Karnaugh maps implicitly) to simplify logic expressions, thereby reducing the number of gates required (e.g., A + A'B simplifies to A + B).
• Common Subexpression Elimination (CSE): Identifying parts of the logic that are repeatedly computed and calculating them only once, then reusing the result. This saves logic resources.
• Dead Code Removal / Constant Propagation: Eliminating logic that does not affect any primary output or that always evaluates to a constant value (0 or 1).
• Resource Sharing: Identifying opportunities to share a single hardware resource (e.g., an adder, a multiplier) across multiple operations that occur at different times or under different conditions. This reduces area but might impact performance.
• FSM (Finite State Machine) Optimization: For state machines, choosing an optimal state encoding (e.g., one-hot, binary, gray code) to minimize logic or improve speed.
• Retiming and Pipelining (Early Stages): The tool may perform architectural transformations like moving registers across combinational logic to balance delays between pipeline stages without altering the design's overall function. This helps in meeting timing constraints by improving critical path delays.

This is the critical step where the optimized, technology-independent logical netlist is translated into a physical implementation using the specific resources available in the target FPGA device or ASIC standard cell library.

• Target Library Integration: The synthesis tool is provided with a "technology library" (or "primitive library" for FPGAs) that contains detailed information about the characteristics of each available physical building block.
• For FPGAs, this library describes the specific LUT sizes (e.g., 6-input LUTs), available flip-flops, dedicated hard IP blocks (DSP slices, Block RAMs, IOBs), and their timing characteristics.
• For ASICs, this library describes the "standard cells" (pre-designed basic gates like NAND, NOR, flip-flops, adders, etc.) provided by the silicon foundry, along with their area, delay, and power consumption.
• "Fitting" Logic to Physical Primitives: The synthesis tool intelligently "fits" the generic logic functions from the previous step onto these specific physical resources. For example, a 5-input Boolean function might be mapped into a single 6-input LUT on an FPGA. A complex adder might be mapped into a series of full-adder gates from an ASIC library, or if available and optimal, directly inferred into a dedicated DSP slice on an FPGA.
• Constraint-Driven Mapping: The mapping process is heavily guided by the design constraints (timing, area, power). If speed is critical, the tool might choose to map logic into faster (potentially larger) combinations of resources or utilize dedicated fast paths. If area is critical, it might combine logic into fewer, more compact resources.

As the final output of the synthesis process, the tool generates a formal gate-level netlist. This is a detailed structural description of the circuit using the actual technology-specific components (e.g., instantiate_LUT6_A from the Xilinx library, AND2X1 from a standard cell library) and explicitly defining all their interconnections.

• Common netlist formats include Verilog netlist (.v or .vg), VHDL netlist (.vhd), or EDIF (Electronic Design Interchange Format).
• This netlist is now a precise blueprint of the digital hardware, ready for the physical implementation stages.

It is crucial to understand that the synthesis process is not purely sequential; it is an iterative and highly optimized flow guided by design constraints specified by the designer. These constraints are typically provided in a separate file, often in a standard format like SDC (Synopsys Design Constraints) or vendor-specific formats like XDC (Xilinx Design Constraints).

• Timing Constraints: The most critical constraints for performance. They define:
• Clock Period/Frequency: The target speed at which the design should operate (e.g., create_clock -period 10ns). This dictates the maximum allowed delay through any combinational path between flip-flops.
• Input/Output Delays: Specify the timing characteristics of external components connected to the FPGA/ASIC pins (e.g., how long it takes for data to arrive at an input pin after a clock edge, or how long it takes for an output to propagate to an external device).
• False Paths: Paths in the design that are logically never active or whose timing is irrelevant for proper operation (e.g., initialization logic, debug paths). The designer explicitly tells the tool to ignore these paths for timing analysis.
• Multi-cycle Paths: Paths that are intentionally designed to take more than one clock cycle to complete. The designer informs the tool about these to relax timing requirements for those specific paths.
• The synthesis tool will aggressively optimize the netlist (by adding buffers, replicating logic, choosing faster gates/LUTs, re-timing) to meet these specified timing requirements.
• Area Constraints: Specify the maximum allowable logic utilization or the number of specific hard IP blocks (e.g., maximum number of LUTs, BRAMs, DSP slices to use on an FPGA, or total gate count for an ASIC). This guides the tool to prioritize smaller implementations.
• Power Constraints: Define target power consumption limits. The tool can employ various techniques (e.g., clock gating inference, operand isolation, selecting low-power cells) to reduce power.
• Pin Assignments: Associate the logical ports defined in the HDL code with specific physical pins on the FPGA package or ASIC die.
• The synthesis tool continuously evaluates the design against these constraints and makes trade-offs. For instance, if a timing constraint is tight, it might use more logic (larger area) to achieve higher speed.