What is High-Level Synthesis (HLS), and how does it differ from traditional HDL design?

High-Level Synthesis (HLS) is an advanced design methodology in digital hardware development that allows you to describe hardware functionality using high-level programming languages like C, C++, or SystemC, instead of traditional hardware description languages (HDLs) like Verilog or VHDL.

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Definition of High-Level Synthesis (HLS)

HLS tools automatically generate synthesizable RTL (Register Transfer Level) code (usually in Verilog or VHDL) from high-level algorithmic descriptions. These tools optimize the design for area, speed, power, and throughput, just like a human RTL designer would.

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Difference Between HLS and Traditional HDL

Aspect	Traditional HDL (Verilog/VHDL)	HLS (C/C++/SystemC)
Design Level	Low-level (cycle-accurate RTL)	High-level (algorithmic/functional)
Language	Verilog, VHDL	C, C++, SystemC
Control Over Timing	Full control over clock cycles and FSM	Abstracted; compiler decides timing
Development Speed	Slower; manual state machine design	Faster; uses software-style coding
Debugging	More difficult; waveform-level	Easier; similar to debugging C/C++
Reuse of Algorithms	Limited	Easy to reuse existing C/C++ code
Optimization Effort	Manual pipelining, unrolling, resource use	Tool-guided via pragmas/directives
Best For	Fine-tuned, performance-critical blocks	Quick prototyping, algorithm-heavy logic

How HLS Works (Flow)

1. Write Algorithm: Describe functionality in C/C++ or SystemC.

2. Apply Pragmas: Guide the tool for loop unrolling, pipelining, etc.

3. HLS Tool (e.g., Vivado HLS):

   • Translates code to RTL.

   • Allows simulation and verification.

4. Export RTL: Resulting Verilog/VHDL is synthesized just like traditional RTL.

5. Implement in FPGA or ASIC.

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Pros of HLS

• Much faster development and iteration.

• Easier debugging and simulation.

• Faster time-to-market.

• Reuse of existing software algorithms.

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Cons of HLS

• Less control over timing and resources.

• May generate inefficient RTL if not guided properly.

• Learning curve for writing hardware-friendly C/C++.

• Not ideal for control-heavy designs like FSMs.

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Popular HLS Tools

Tool	Vendor	Language Support
Vivado HLS / Vitis HLS	Xilinx	C, C++
Intel HLS Compiler	Intel (Altera)	C++
Catapult HLS	Siemens (Mentor)	SystemC
LegUp	University of Toronto	C

 Example

C (HLS style):

c

void add(int A[100], int B[100], int C[100]) {
#pragma HLS PIPELINE
  for (int i = 0; i < 100; i++) {
    C[i] = A[i] + B[i];
  }
}

Equivalent Verilog (simplified):

verilog

always @(posedge clk) begin
  if (valid) begin
    C[i] <= A[i] + B[i];
    i <= i + 1;
  end
end

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 Summary

• HLS allows you to design hardware using software-like languages and methods.

• It's best for algorithm-heavy or data path-focused blocks.

• It does not replace RTL in every case—RTL is still better for finely optimized or control-heavy logic.