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When S3 is turned on (FX2 PON), the 1.2 V and 2.5 V power rails are controlled by the USB (EZ-USB FX2LP USB FX2) microcontroller. At start-up, the USB microcontroller switches off the 1.2 V and 2.5 V power rails and starts up the module in low-power mode. After enumeration, the USB microcontroller firmware switches the 1.2 V and 2.5 V power rails on, if enough current is available from the USB bus.

When S3 is turned off, the 1.2 V and 2.5 V power rails are always enabled.

	When S3 is turned on (FX2 PON), make sure that no signals are applied to the input pins when power-rails are disabled by the USB microcontroller (at start-up).

S3 position	Default position	Effect on 1.2 V and 2.5 V rails
FX2 PON (on)		1.2 V and 2.5 V rails controlled by USB FX2 microcontroller (signal FX2_PS_EN) PS_EN = FX2_PS_EN = 1 or 0
PON (off)		1.2 V and 2.5 V rails always enabled (PS_EN = 1) PS_EN ≠ FX_PS_EN = 1 or 0

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