For XC9500 family devices, a Fitter option allows me to configure unused I/O as programmable ground pins (PGND) or leave them floating (TIE). Can I selectively define the TIE/PGND mode for individual pins?
As of 3.1i Service Pack 1
When unused I/Os are assigned to programmable ground and the PROHIBIT constraint is assigned to particular pins, those pins with the PROHIBIT constraint are not grounded and will float.
The UCF syntax for setting a PROHIBIT constraint is as follows:
- For pinned packages, replace "xx" with the pin number:
CONFIG PROHIBIT Pxx;
For example: CONFIG PROHIBIT=P12;
- For ball-grid packages, replace "xx" with the alphanumeric pin number:
CONFIG PROHIBIT xx;
For example: CONFIG PROHIBIT=A12;
For Version 2.1i
- Turn off the option to configure unused I/O as programmable grounds.
- Ground the pins that you want to ground (PGND) in your design.
- Use the PROHIBIT constraint on the signals you want to TIE. This instructs the Fitter not to use these pins. When you elect not to use unused I/O as programmable grounds, these pins will float.