How long after the Program pin is released is the JTAG TAP available when I configure a Virtex, Virtex-E, or Spartan-II device?
If you hold the Virtex PROG pin low, this will force the TAP into the Test-Logic-Reset (TLR) state.
The JTAG TAP is available immediately after the Program pin is released. At this time, the IDCODE instruction is loaded according to the JTAG standard. If a configuration instruction is to be loaded, wait 100 us to allow the device to finish clearing the configuration memory.
For more information, please refer to the Power-up Timing Characteristics in the data book for the appropriate device.