When I use the schematic capture tool to add a value to the PLL attribute CLKIN_PERIOD in ECS, I receive the following message:
The value I specified is not assigned to CLKIN_PERIOD.
The attribute traits of CLKIN_PERIOD show that both the upper bound and lower bound are "0," and I cannot change these values.
This issue is Fixed in ISE 10.1.
For ISE 9.2i and earlier, you can work around this problem by specifying the CLKIN_PERIOD attribute in the UCF.
The basic UCF syntax is:
where "n" is a floating-point number representing nanoseconds.
The following statement specifies a 35-nanosecond period of the clock used to drive the CLKIN pin of the PLL: