188.8.131.52. Overriding I/O Placement Rules on Differential Pins
I/O placement rules ensure that noisy signals do not corrupt neighboring signals. Each device family has predefined I/O placement rules.
I/O placement rules define, for example, the allowed placement of single-ended I/O with respect to differential pins, or how many output and bidirectional pins you can place within a VREF group when using voltage referenced input standards.
Use the IO_MAXIMUM_TOGGLE_RATE assignment to override I/O placement rules on pins, such as system reset pins that do not switch during normal design activity. Setting a value of 0 MHz for this assignment causes the Fitter to recognize the pin at a DC state throughout device operation. The Fitter excludes the assigned pin from placement rule analysis. Do not assign an IO_MAXIMUM_TOGGLE_RATE of 0 MHz to any actively switching pin, or your design may not function as you intend.