Logic Design for Array-Based Circuits

Copyright © 1996, 2001, 2002, 2008, 2016 Donnamaie E. White , WhitePubs Enterprises, Inc.

• Table of Contents
  
• Preface
  
• Overview
  
• Chapter 1 Introduction
• Chapter 2 Structured Design Methodology
  
  • Structured Design Methodology
  • Review of the Available Arrays
  • Initial Sizing of the Circuit
  • Create the Preliminary Macro Schematic (when schematics are used)
  • Compute the Path Propagation Delay
  • Compute the Estimated Power
  • Pre-Simultation Steps
  • Simulation
  • Fault Grading
  • Design Submission Through Prototype
    • Design Validation Review
    • Placement
• Chapter 3 Sizing the Design
• Chapter 3 Appendix = Case Study in Sizing a Design
• Chapter 4 Design Optimization
• Chapter 5 Timing Analysis for Arrays
• Chapter 6 External Set-up and Hold Times
• Chapter 7 Power Considerations
• Case Study: DC Power Computation
• Case Study: AC Power Computation
• Chapter 8 Simulation
• Case Study: Simulationest
• Chapter 9 Faults and Fault Detectiond Output
• Chapter 10 Design Submission
• ASIC Glossary
  • Glossary A-D
  • Glossary E-K
  • Glossary L-R
  • Glossary S-Z
  • Symbols

Structured Design Methodology

Compute the Path Propagation Delay

Compute the path propagation delay for the most critical (time sensitive) paths in the circuit. Make adjustments to the schematic in terms of macro options for speed where needed. Does the estimated performance satisfy the specification?

There may be different multipliers for the different product grades, Commercial and Military, and for different power supplies within the product grade. The multiplier may depend on the macro type.

Many arrays are specified without worst-case timing multipliers. They are specified with min/max ranges for each macro propagation delay. Maximum path delay is found using the MAX data although the conditions for a maximum propagation delay for an individual macro will vary. Minimum delays are found using the MIN data.

Be certain that the proper fan-out loading and performance specifications are selected when doing this computation. Because of the high degree of variation in the way a library is documented between vendors and between array series from the same vendor, be certain that the rules regarding the methods of specifying timing delays for the macros for the array series selected are clearly understood.

Internal extrinsic loading delays are composed of metal load (Lnet), electrical fan-out load, the sum of all loads driven (Lfo), wire-OR electrical loading if the array allows wire-ORs and if one was used in the net (Lwo) and the k-factors for each. The k-factors, expressed in ns/LU, convert the load units into time units. Table 2-7 shows the extrinsic load equations for internal nets as they are used by AMCC and other vendors. K-factors may be specified as tables, graphs, or broken down into parts for temperature, voltage and processing. Check with the specific vendor.

Table 2-7 Components Of Path Delay - Internal Loading

External extrinsic loading delays are composed of the system load capacitance and the package pin capacitance (Lcap) and the k-factor. The k-factor, expressed in ns/pF, convert the load capacitance into time units. The equation used by AMCC for this delay are listed in Table 2-8.

All this became moot when first RTL (VHDL then Verolog) came into being followed by Design Compiler and Design Analyzer, and then PrimeTime STA software. Static Timing Analysis tools compute the timing, find the worst-case path, examine set-up and hold, compute path propagation delays, worst case (if you tell them to), etc. and took over the tedious and error-prone methods those of us who designed in the bipolar array era pre-RTL had to perform by hand.

Understanding the methodology however is still valid, and allows today's designer to understand just what is being computed by these EDA software tools. Too often, the designer runs them, but doesn't "get" them.

Copyright © 1996, 2001, 2002, 2008, 2016 Donnamaie E. White , WhitePubs Enterprises, Inc.
For problems or questions on these pages, contact [email protected]