Please make sure of the following :
(1) RESET wired to provide a reset time of 10 ms (recommended)
(2) Make sure that the CLOCK is stable for at least 200 us before RESET is released
As the SX2 performs some self initialization on power-up. A part of this initialization is to set the DISCON bit so that the SX2 device shows up disconnected. If, during the power on sequence, the SX2 is held in Reset for a long period, it can be recognized by the Host Controller as an Unknown Device, and can be dropped off of USB. In normal operation the SX2 will disconnect itself from USB after Reset, awaiting the command from the external master to enumerate. If, however, the SX2 is held in Reset after power on it will be halted from performing the actual disconnect (setting the DISCON bit). If this connected, but not yet operational state exists for too long, dependent on Host conditions, the Host will identify the SX2 as a Unknown Device. A reset time of 10 ms is adequate and recommended for the SX2. We recommend that one use a reset time of 10 ms with an RC network (100K/0.1 uF).
Also, when power is applied to the SX2, the Reset circuit holds the SX2 chip in Reset briefly until the PLL (Phase Lock Loop) locks to the 24 MHz oscillator. Once the PLL stabilizes, the SX2 chip emits a stable CLKOUT 48Mhz. The SX2 must be held in reset until the PLL stabilizes. Test results have shown that the CLOCK must be up and stable for at least 200 us during reset to be a good working solution. Make sure that the clock is running for at least 200 us before de-asserting the RESET.