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
||# I/O Cells Required
TOTAL I/O CELLS: 153
||# L Cells Required
TOTAL L CELLS: 245
The number of macros is not the same as the number of cells, even for
the I/O macros.
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
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