Logic Design for Array-Based Circuits

by Donnamaie E. White

Copyright © 1996, 2001, 2002 Donnamaie E. White

 

Case Study: Sizing A Design

Last Edit July 22, 2001


REVIEW OF SIZE - SECOND PASS

The revised estimate (one version of the solution) shows the circuit requirements as they are now understood.

Table A-3 Second Sizing Estimates

Number of Cells Required

#macros
MACRO
CELLS
TOTAL
79
IE93S
1
79
73
OE42S
1
73
1
IE31H
1
2
9
IEVCC
1
9

TOTAL I/O CELLS REQUIRED 162

10
MX21S
2
20
11
GT60S
3
33
10
GT09S
1
10
4
GT55D
2
8
2
GT87D
2
4
32
FF10S
3
96
32
FF46S
3
96

TOTAL L CELLS REQUIRED 267

Change OE42S to OE11S and delete the 2 GT87Ds.

This fits into the Q20080 array that has 162 I/O cells and 2044 L cells. This is a severely I/O-bound design (of course!). A design is either core-limited or I/O limited.

Note: When vectors are written for this array, they should be designed so that no more than 16-32 of the outputs switch at any one time. These are AMCC-specific vector design rules.

Table A-4 AMCCERC Population ERC



PACKAGE SIZE

The minimum number of signal pins that should be available on a package for this circuit is 157 (162 signals plus the 4 fixed signals minus the 9 added grounds). The worst-case number of signal pins that could be required on a package for this circuit is 166 (162 signals plus the 4 fixed signals). The truth is in the middle and is placement-dependent.

PROBLEMS

  • The OE42S is limited to a toggle frequency of 350MHz. If the clock is running at 500MHz, the outputs could be toggling slower. If not, then the OE42S is not a correct choice if speed is to be maintained. Neither is the OE11S!

  • Insufficient added grounds is not a minor problem.

  • The circuit uses nearly 8 Watts - much too high.

ALTERNATIVE SOLUTION

The differential output OE14S could be used in place of two OE42S macros and the GT87D driver (at least one) could be deleted. This reduces the OE42S macros from 73 to 9, and the 7 always-on enables could be driven by a GT08L NOR gate instead of a static driver macro.

The use of OE14S provides a cleaner solution (less skew) plus it frees internal cells. The maximum frequency of the OE14S is 1.2GHz. One output pad can be used as the true signal and the other as the compliment.

Another advantage is the reduced requirement for added grounds. The 32 differential outputs count as 32 outputs and not as 64, reducing the re-quirement for this group to 8 added IEVCC, what was provided. The ninth IEVCC applies to the miscellaneous other outputs. There will be a warning issued by AMCCERC that there might not be sufficient added grounds for these miscellaneous outputs - the algorithm defined by AMCC requires that two IEVCC macros be added.

Table A-5 OE42S Solution
Table A-65 OE14S Solution
IE93S
78
IE93S
78
OE42S
73
OE42S
9
 
 
OE14S
32
IE31H
1
IE31H
1
IEVCC
9
IEVCC
9
MX21S
10
MX21S
10
GT87D
2
GT87D
1
GT60S
11
 
 
GT09S
8
GT09S
8
GT55D
4
GT55D
4
FF10S
32
FF10S
32
FF46S
32
FF46S
32

POWER

The DC power dissipation for the maximum worst-case MILITARY DC power for the OE42S version of the circuit was estimated to be over 8 Watts.

The DC power computation for the OE14S version, same conditions, is esti-mated to be 5.88 Watts. (This number is based on the circuit as shown in the schematics and the February 1991 library specifications.)

Reducing the GT08S macros to GT08L macros can further reduce power.

 

 

 

 

 

 

 

 

 

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