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AR# 20546

Virtex-4, I/O - What is the capacitance of LOW capacitance I/O? How much lower is it than NORMAL?

Description

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 ?

Solution

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.

AR# 20546
Date Created 09/03/2007
Last Updated 12/15/2012
Status Active
Type General Article