Logic Design for Array-Based Circuits

by Donnamaie E. White

Copyright © 1996, 2001, 2002 Donnamaie E. White

 

Structured Design Methodology

Last Edit July 22, 2001


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?

Sum of Macro Sum of Macro
Path Delay = Intrinsic Delays + Extrinsic Loading
Delays

For the arrays that use typical specifications, be certain to use the correct multiplication factor (WCM) for this worst-case analysis. Review the assumptions made in establishing the multiplication factors and adjust them if these assumptions are not expected to be met (i.e., derate the performance by a higher factor). Some vendors call these multiplication factors "adjustment factors". Be clear as to what is being adjusted and why.

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.

Will the array support the maximum frequency
of operation and the critical path
performance requirements?

 

Table 2-7 Components Of Path Delay - Internal Loading

General Equation for Internal Extrinsic Delay:

No wire-OR allowed:

tex = knet * Lnet+ kfo * Lfo

General Equation for Internal Extrinsic Delay:

Wire-OR allowed:

tex = knet * Lnet+ kfo * Lfo + kwo * Lwo

Worst-case Internal Extrinsic Delay:

For Arrays with a Worst-Case Multiplier:

texwc = WCM * tex

For Arrays with no Worst-Case Multiplier:

tex is already worst-case

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.

Table 2-8 Components Of Path Delay - External Loading

General Equation for External Extrinsic Delay:

tex = kcap * Lcap

Worst-case Internal Extrinsic Delay:

For Arrays with a Worst-Case Multiplier:

texwc = WCM * tex

For Arrays with no Worst-Case Multiplier:

tex is already worst-case

 

 

Copyright @ 2001, 2002 Donnamaie E. White, White Enterprises
For problems or questions on these pages, contact dew@Donnamaie.com