Answers Database

XC9500/XC7300: How to preserve/keep/save or prohibit software from using certain pins


Record #1987

Problem Title:
XC9500/XC7300:  How to preserve/keep/save or prohibit software from
using certain pins



Problem Description:
Keywords:  9500, pins

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.  With the
current CPLD software, there is no way to tell the software to
reserve given pins.  One must create a dummy circuit to 'trick'
the software into thinking that the pin is actually used.


Solution 1:

One possible solution:
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