When the FX1/FX2/FX2LP comes out of reset there should be some pulses on the CLKOUT signal. If the CLKOUT signal never turns on, the part has not properly started. This will result in enumeration failure. Items that should be verified are:
VCC pins have 3.0 to 3.6 volts applied to them.
AVCC pin has 3.0 to 3.6 volts applied to them.
|The crystal is oscillating. If the crystal does not oscillate verify the load capacitors. If the capacitors are the proper load capacitors for the crystal (recommended value is a 22 pF capacitor on a crystal requiring 20-24 pF load capacitor) verify the drive level of the crystal (it should be 500 mW or greater)|
|4)||Verify the pull-up resistors on the SDA and SCL lines are in the range of 1K to 20 K ohms (recommended value is 2.2 K ohms) and are installed correctly.|
|5)||Verify the RESET signal has a valid period after valid power has been reached. If you are using a slow turn on regulator an RC circuit may charge at the same rate power is turning on. If this occurs the part will not receive a valid reset.|