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 STATUS SO FAR

The first sizing estimate provides the cell counts shown in Table A-2.

Table A-2 First Sizing Estimates

# MACROS Macro # I/O Cells Required
78 IE93S 78
73 OE42S 73
1 IE31H 2

TOTAL I/O CELLS: 153

# MACROS Macro # L Cells Required
10 MX21S 20
11 GT60L 33
32 FF10S 96
32 FF46S 96

TOTAL L CELLS: 245

The number of macros is not the same as the number of cells, even for the I/O macros.

Exercise

Check the cell counts against the current design manual for the Q20000 Series. Check for new MSI macros or new I/O macros that might be used in place of those selected (such as OE11S). Consider size, speed and power in making changes. (Changes should be made!)

If you are designing with a different array series, create the same table for the chosen library.

SIMULTANEOUSLY SWITCHING OUTPUTS

Since 64 outputs are switching simultaneously in the worst case (master reset is one example), additional IEVCC macros (added ground) will be required according to the Q20000 Series design rules. A total of 16 IEVCC macros is required for these outputs and each blocks off one I/O cell and uses one pad.

This is the minimum number of added power and grounds recommended for worst-case conditions.

Adding two more outputs for the 16:1 MUX Y outputs, six for the pass-through and one for the gate tree, requires two more IEVCC macros.

  • If the outputs switch within one macro delay (or within 2 ns, whichever is larger) of the other switching group additional IEVCC is required.
  • If they switch well separated in time from the other group, then the added IEVCC for this group will not be required.

By tagging the switching groups and the added power and ground macros that belong to the groups with a SWGROUP parameter or property, the AMCC MacroMatrix can check for sufficient added power and grounds.

For this design, assume that the groups are not simultaneously switching more than 32 signals, allowing a reduction in added ground. Allowing 8 IEVCC for the pipeline outputs (switching group AAA) and one for the rest of the circuit (switching group BBB), nine IEVCC macros are required.

Adding these 9 IEVCC macros to the previous counts (153 + 9), the number of I/O cells used is 162. This is exactly the number of I/O cells available for circuit use on the Q20080 array. (This does not count the four fixed I/O signals for the AC Speed Monitor and the thermal diode that have pre-assigned PADs.) The added ground macro is shown in Figure A-9.

Figure A-9 Added IEVCC Macro

Note: Using less than the recommended number of added grounds is not a good idea. It will require engineering approval before design submission and could cause other problems later. Think about another solution!

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

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