digital logic design

🔧 Digital Logic in Schematic-to-GDSII (Layout) Flow – Overview
The schematic-to-GDSII flow is a standard methodology in custom VLSI design, where digital logic is implemented at the transistor level and transformed into a physical layout for fabrication. This flow bridges the gap between logical behavior and physical implementation.

🔹 1. Digital Logic Design (Boolean Level):
The design begins by defining the logic functionality using Boolean expressions or truth tables.
Logic gates such as AND, OR, NOT, XOR, NAND, NOR are chosen to construct combinational or sequential logic circuits.
This logic may be derived manually or through RTL (Register Transfer Level) synthesis, but in custom layout design, we often implement at transistor level for optimization.

🔹 2. Transistor-Level Schematic Creation:
Each logic gate is implemented using CMOS technology (complementary pairs of NMOS and PMOS transistors).
A pull-up network (PMOS) connects outputs to VDD, and a pull-down network (NMOS) connects outputs to GND, ensuring correct logic levels.
The complete digital circuit is built by interconnecting these transistor-level gates.

🔹 3. Simulation & Verification:
The schematic is simulated to validate the logic using SPICE or a digital simulator.
Functional correctness and timing behavior are verified before moving to layout.

🔹 4. Layout Design (Custom Physical Design):
Transistors from the schematic are placed and routed manually in a layout editor.
Interconnects are made using metal layers following design rules from the technology node (e.g., 180nm).
Optimization for area, symmetry, parasitic minimization, and power routing is done.

🔹 5. DRC (Design Rule Check):
Ensures the physical layout complies with the foundry’s geometric constraints (widths, spacings, enclosures, overlaps).
DRC errors must be resolved to ensure manufacturability.

🔹 6. LVS (Layout vs. Schematic):
Compares the netlist extracted from the layout with the original schematic.
Ensures electrical equivalence and catches issues like shorts, opens, or incorrect connectivity.

🔹 7. Parasitic Extraction (PEX):
The layout is analyzed for real-world effects such as resistance and capacitance introduced by wires and diffusion.
Extracted netlist includes parasitics and is used for post-layout simulation.

🔹 8. Post-Layout Simulation:
Validates the design under realistic conditions, capturing delays, glitches, or crosstalk.
Confirms timing margins and logic correctness before fabrication.

🔹 9. GDSII Generation:
The final layout is exported as a GDSII file, which contains mask-level information.
This file is sent to the semiconductor foundry for chip fabrication.

✅ Summary:
The schematic-to-GDSII flow takes digital logic from abstract Boolean expressions to a fabrication-ready physical design. It involves careful transistor-level implementation, layout creation, and rigorous verification through DRC, LVS, and PEX. This flow is foundational to custom IC design, enabling designers to optimize for area, power, performance, and reliability.