The extended Spartan-3A device families have a VCCO2T threshold requirement of 2.0V minimum; therefore, they cannot have VCCO_2 at 1.8V during configuration. However, VCCO2T does not apply after configuration; therefore, VCCO_2 can be lowered to 1.8V or lower after DONE goes High.
How do I lower VCCO_2 after configuration?
To lower VCCO_2 after configuration, use an adjustable regulator with a feedback loop to set the output voltage; use a resistor divider network to define the voltage values. One resistor would be connected to an I/O which is disabled before configuration (using PUDC_B High) and Low after configuration (driving it to HIGH in the design). In the following figure, R1 in series with R2 would set the regulator output voltage at 2.0V or higher for power-on and during configuration. Resistor R3 is enabled after configuration, and the parallel resistance of R1 and R3 would replace R1 to set the regulator voltage at 1.8V or lower after configuration.
NOTE: The Spartan-3 Generation Configuration User Guide (UG332) v1.5 instructs you to drive the I/O to GND in the design:
However, this is incorrect. You should drive the I/O to HIGH.
This error is scheduled to be fixed in v1.6 of the Spartan-3 Generation Configuration User Guide (UG332).