Why is the USB-Serial device not recognized after enabling "VDD less than 2 V" in the Configuration Utility?
The 1.8-V internal regulator is enabled by default in USB-Serial bridge controllers. If you are supplying 1.8 V as VDDD/VCCIO, make sure you also connect the VCCD pin of the device to 1.8 V, to bypass the regulator. Then, select “VDD less than 2 V” in the Configuration Utility.
When the VCCD pin is connected to 1.8 V, the USB-Serial bridge controller enumerates.