What is the capacitance of LOW capacitance I/O? How much lower is it than NORMAL?
What is the difference when setting CAPACITANCE attribute LOW, NORMAL or DONT_CARE ?
CAPACITANCE is a new I/O attribute available for Virtex-4 I/O.
When set to LOW, the ISE tool will place the I/O to pins designated as "LC" in the package pin-out.
When set to NORMAL, the ISE tool will place the I/O to "non-LC" pins.
When set to DONT_CARE(default), the ISE tool will place the I/O to any available pins.
The "LC" I/O do not have differential output (LVDS_25, LVDSEXT_25, LVDS_25_DCI, ULVDS_25, and LDT_25).
The 'LOW' capacitance setting is only slightly lower (approximately 0.5 pF) than the 'NORMAL capacitance I/O. Therefore, it is not significant enough to impact design interface. Some 'LC' pins are either clock capable or global clock pins, and thus can be used to input clocks using dedicated clock routing.