What is Synthesis - And How Does RTL Become Actual Gates in a Chip?

So far we’ve learned:

• What a chip is
• What an SoC is
• What RTL is
• What Verification is

At this stage, we have a valid logical description of the hardware - but it’s still just text in a language like Verilog or SystemVerilog.

For the chip to become something real, we must go through another stage:

Synthesis Or: translation of RTL to physical logic.

This is the first stage where the hardware approaches reality.

What Actually Happens in Synthesis?

The process can be described as follows:

1. Take the RTL
2. Add timing and power consumption constraints
3. Run the tools
4. Get a Netlist - a list of actual gates:
   • AND, OR, NOT, MUX, flip-flops, and more

This is not code - this is actual hardware at the logical level.

The Netlist is already a structure that later becomes transistors in Backend.

Why Can’t We Directly Manufacture a Chip from RTL?

RTL is a behavioral description. It describes what should happen - not how to build it physically.

Example:

In RTL you can write:

if (a > b) out = a;
else out = b;

But in an actual chip, there’s no “if”. There are:

• Multiplexers
• Comparators
• Gates
• Paths

Synthesis takes easy-to-understand commands - and turns them into an actual physical circuit.

What Decisions Are Made During Synthesis?

Deep beneath the surface, the process decides:

• Which gates to use
• What the width and depth of paths will be
• How to balance speed and power consumption
• How to meet timing requirements
• How to consolidate redundant logic
• Whether there are duplications that can be removed

Synthesis tools try to “optimize” the design - just like a software compiler, but for the physical world.

Constraints - What Turns RTL into Something Real

To direct the result, constraints are added:

Clock Frequency

For example: “The component must work at 1GHz”.

The tool will try to build paths fast enough to meet this.

Timing Constraints

For example: “This signal must arrive on time to another section”.

Power Consumption Constraints

Sometimes speed is sacrificed for energy savings.

Interface Connections

Precise definition of signals coming from outside.

Constraints are what direct Synthesis to the right result.

What Comes Out in the End?

The synthesis output is:

Netlist A file that describes:

• Which gates exist
• How they’re connected
• How to implement each physical operation

This is the “actual hardware” at the logical level.

In the next stage, Backend teams will work on the Netlist.

An Analogy to Clear Things Up

Think of Synthesis as a translation process:

High-level description: “Build me a house with two rooms, a living room, and a kitchen.”

Synthesis: “I translate this into bricks, beams, concrete, and plumbing.”

RTL says what should happen. Synthesis determines what it’s actually made of.

Why Is Synthesis So Important?

Because it:

• Reveals timing problems
• Reveals impossible logic
• Allows discovering bugs not seen in simulation
• Creates the foundation for Backend operations
• Enables seeing the hardware “crystallize” for the first time

First time the chip takes tangible form.

Where Do We Go From Here?

After the Netlist is created:

1. Pass it to the Backend team
2. Start Place & Route
3. Handle actual timings
4. Build the chip’s physical layout
5. And prepare it for Tapeout

In other words:

Synthesis is the bridge between the logical description world - and the actual physical world.

Summary

Synthesis is a stage where:

• RTL is translated to actual gates
• Behavior becomes hardware
• Constraints determine design quality
• Netlist is the output leading to manufacturing
• The chip starts to “be born” for the first time

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In the next post, we’ll learn about Place & Route - how to position gates on the chip and connect them with actual wires, and how this becomes a complete physical layout.