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 A3 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/Obound design (of course!). A design is either corelimited
or I/O limited.
Note: When vectors are written for this array, they should be designed
so that no more than 1632 of the outputs switch at any one time. These
are AMCCspecific vector design rules.
Table A4 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 worstcase 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 placementdependent.
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 alwayson 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 requirement
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
A5 OE42S Solution

Table
A65 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 worstcase 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 estimated
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.
