What is the resistance value of the IOB pull-ups and pull-downs in Spartan-II devices? Why isn't this information in the Spartan-II data sheet?
The Spartan-II data sheet lists the maximum current when internal pull-ups and pull-downs are used (http://www.xilinx.com/support/documentation/data_sheets/ds001.pdf,page 2, Irpu & Irpd).
These values were taken from a device sample, and indicate a minimum pull-up resistance of 13.2K ohms and a minimum pull-down resistance of 24K ohms.
The pull-up and pull-down strengths are not expressed in terms of precise resistive values in the data sheet because these values can vary from batch to batch. If a precise value is required, you should not rely upon the internal pull-ups or pull-downs.