Do CoolRunner devices have weak pull-up resistors on their I/Os?
CoolRunner XPLA3 devices have pull-up resistors on the I/O pins, which can be activated on a pin-by-pin basis for inputs only. CoolRunner-II devices also have pull-up resistors on the I/O pins. For further details, see (Xilinx Answer 14318). You can configure these in the Xilinx Constraints Editor or in a User Constraints File (UCF). The UCF syntax is as follows:
NET signalname PULLUP;
These pull-up resistors can also be used on unused I/O to prevent unused pins from floating:
- In the 5.1i software, this option is located under the "Fit" process -> "Basic" tab -> "Unused Pin Termination".
- In the 4.1i software, this option is located under the "Generate Programming File" process.