Answers Database

M1.4 CPLD: How to reserve a pin or a macrocell for future use?


Record #2906

Product Family:  Software

Product Line:  CPLD Implementation

Problem Title:
M1.4 CPLD: How to reserve a pin or a macrocell for future use?


Problem Description:
Keywords: PROHIBIT, reserve pin, reserve macrocell

Urgency: standard

General Description:

The PROHIBIT constraint is not currently supported for CPLDs. In order to
reserve a pin(s) or a macrocell(s) redundant logic has to be used.


Solution 1:

In order to keep the Fitter from using the pin or macrocell that you want to
reserve for future use, do the following:

Schematic Users -

Drive the pin that you want to reserve with a GND symbol. Make sure that there
is an OBUF between the GND symbol and the Output pad. Now put a LOC constraint
on this PAD, either in your schematic or using an UCF file. Compile your
design normally. In the 'Implemented Equations' section of the Fitter report,
you can see that the pin you wanted to reserve is now being driven by GND.

When you want to use this pin in future, just remove the GND symbol and connect
your logic to this pin.



End of Record #2906