Answers Database


How to preserve/keep/save or prohibit software from using certain pins


Record #1987

Problem Title:
How to preserve/keep/save or prohibit software from using certain pins


Problem Description:
Urgency: Standard

General Description:

It is sometimes desired to reserve particular I/O pins on the
device for future use. You may not yet have logic connected to
these pins in the design, but you still do not want the
software to use these pins for any other signals.


Solution 1:

There is an attribute called PROHIBIT which performs this
exact functionality. This constraint is available for all families.
In the UCF, add a line for each pin that you wish to preserve.
Example:
CONFIG PROHIBIT = P26;



Solution 2:

An alternate solution if you wish to avoid use of the UCF:
To preserve a given I/O pin, use the following circuit:
		 _______________
		|	       |
		|	       |
	   _____/|_____        |
	  |	\|    |       ---
 _____	  |    OBUFE  |        GND
<IOPAD>---|	      |
 -----	  |	      |
LOC=P25   |_____|\____|
		|/
	       IBUF

The software will not trim this logic out, since it does not
see the logic associated with the pin as being sourceless or
loadless.

Also, since it is "used," it will not route any other signals
to this pin. This is in effect, permanently 3-stating the pin.

Apply the appropriate LOC attribute to preserve the desired pin.




End of Record #1987 - Last Modified: 01/19/00 16:38

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