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Design Abstractions ​

Integrated circuit (IC) design is built upon multiple abstraction levels, each providing a different view of the system. These abstractions allow designers to handle complexity, moving from a high-level functional description down to the*physical layout ready for fabrication.

Why abstractions matter ​

  • They allow designers to focus on the right level of detail at each stage.
  • They separate functionality from implementation details.
  • They provide a common language between different design tools and flows.

Abstraction levels in digital design ​

  1. Behavioral level

    • High-level description of functionality (e.g., algorithms, state machines).
    • Typically expressed in languages such as SystemVerilog, VHDL, or even C/SystemC for modeling.

  2. Register Transfer Level (RTL)

    • Describes data flow between registers and logic operations.
    • Synthesizable into gate-level circuits.
    • Example: Verilog RTL for an adder or a finite state machine.

  3. Gate level

    • Netlist of standard cells (AND, OR, flip-flops, etc.) mapped to a target library.
    • Generated by synthesis tools (e.g., Yosys).

  4. Transistor level

    • Actual CMOS implementation of each standard cell.
    • Standard cell libraries provide these circuits.

  5. Physical layout

    • Placement and routing of cells, interconnect, and power networks.
    • Exported as GDSII for fabrication.

Abstraction levels in analog design ​

  1. System level

    • Block diagrams describing overall functionality (e.g., LDO regulator, amplifier stages).

  2. Schematic level

    • Circuits drawn with transistors, resistors, capacitors, etc.
    • Designed and simulated in tools like Xschem or Ngspice.

  3. Transistor/device level

    • MOSFET and BJT models with parameters from the PDK.
    • Includes device sizing and biasing.

  4. Small-signal models

    • Linearized representations for AC, noise, and stability analysis.

  5. Physical layout

    • Transistor geometries, interconnections, matching techniques.
    • Verified with DRC/LVS before tapeout.

Digital vs. Analog perspectives ​

  • Digital design benefits from automation (synthesis, place & route).
  • Analog design still relies heavily on manual design decisions (topologies, sizing, layout techniques).
  • Both domains converge in mixed-signal design, where analog and digital abstractions coexist.