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

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Scroll pdf title
title Slide switch S3 settings overview ( power rails 1.2 V and 2.5 V only)
S3 position Default position Effect on 1.2 V and 2.5 V power rails FX2 PON (on, closed)  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, open) 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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Signal FX2_PS_EN

To command signal FX2_PS_EN, read the reference firmware code.
IOD = 0x03; // Enable PS_EN and disable PROG_B
OED = 0x03; // Configure PS_EN and PROG as outputs

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When slide switch S3 is in the left position ( = FX2 PON : power rails conditionally on depending on signal FX2_PS_EN), signal PS_EN is set to signal FX2_PS_EN (PS_EN = FX2_PS_EN) driven by the EZ-USB FX2LP USB FX2 microcontroller under user control (IOD and OED of fw.c). 

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1. Dynamic full power operation (PS_EN = 1): when the EZ-USB FX2LP USB FX2 microcontroller sets signal PS_EN = FX2_PS_EN = 1, power rails 1.2 V and 2.5 V are enabled.
   This setting can be useful for .
2. Dynamic low power operation (PS_EN = 0): when the EZ-USB FX2LP USB FX2 microcontroller resets signal PS_EN = FX2_PS_EN = 0, the following components are switched off:

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The table below summarizes all switching options implied by slide switch S3 and firmware signal FX2_PS_EN (under the standard assembly option). 

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