Annotations SPEF

Parasitic Elements in VLSI Design

What are Parasitic Elements?

• Parasitic elements are unintended circuit elements that negatively affect chip performance.
• Also known as non-ideal elements, they result from physical limitations of fabrication.

Types of Parasitic Elements

• FEOL (Front-End of Line): Parasitics at the transistor level.
• MEOL (Middle-End of Line): Parasitics at the contact level (drain-to-source).
• BEOL (Back-End of Line): Parasitics in the interconnect layers (PnR level).

Parasitic Extraction (PEX)

PEX refers to extracting parasitic info like:

• Resistance (R)
• Capacitance (C)
• Inductance (L)

Data is extracted from the layout using EDA tools.

Parasitic File Formats

1. DSPF (Detailed Standard Parasitic Format):
   • Highly detailed info, useful for SPICE simulation.
   • Captures every segment of a net.
   • Large file size, high accuracy.
2. RSPF (Reduced Standard Parasitic Format):
   • Simplified version, faster analysis.
   • Lower accuracy, better for large designs.
3. SPEF (Standard Parasitic Exchange Format):
   • Compact format of detailed parasitic data.
   • Supports best, typical, and worst-case values.
   • Commonly used in STA and IR analysis.

EDA Tools That Support SPEF

• Quantus – Cadence
• StarRC – Synopsys
• Calibre xACT 3D – Mentor/Siemens

SPEF File Structure

SPEF files have four main parts:

1. Header Section: Includes design name, tool, naming styles, and units.
2. Name Map Section: Shortens long net names.
   Example:
   *509 F_C_EP2
*510 F_C_EP3
*511 F_C_EP4
3. Port Section: Lists input/output/bidirectional ports.
   Example:
   *PORTS
*1 I
*2 I
*3 O
*4 O
4. Parasitic Data Section: Each net has a *D_NET block.
   Example:

   *D_NET regcontrol_top/GRC/n13345 1.94482
   *CONN
     *I regcontrol_top/GRC/U9743:E I *C 537.855 9150.11 *L 3.70000
     *I regcontrol_top/GRC/U9409:A O *C 540.735 9146.02 *D 5.40000
   *CAP
     1 regcontrol_top/GRC/U9743:E 0.936057
     1 regcontrol_top/GRC/U9409:A regcontrol_top/GRC/U10716:Z 0.622675
   *RES
     1 regcontrol_top/GRC/U9743:E regcontrol_top/GRC/U9407:Z 10.7916
   *END
       

Notes on Sections

• *CONN: Shows pin connections. Pins can be to instances (*I) or ports (*P).
• *CAP: Lists capacitance values – lumped or coupling.
• *RES: Lists resistance between nodes.