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title Dip switch S1[A:D aka 1:4]. The switch S1B is the one with label 2 and B

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title When Dip switch S1B = FX2 PON, signal PS_EN is set to signal =FX2_PS_EN driven by the EZ-USB FX2LP USB FX2 microcontroller under user control.

When S1B is turned off (open)   (FX2 PON position), the 1.2 V, 1.5 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 power rails 1.2 V, 1.5 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, 1.5 V and 2.5 V, if enough current is available from the USB bus.

When S1B is turned on (closed) (PON position), the power rails 1.2 V, 1.5 V and 2.5 V are always enabled (switched on).

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title Dip switch S1B settings overview (power rails 1.2 V, 1.5V and 2.5 V only).
S1B position Default position Effect on 1.2 V, 1.5 V and 2.5 V rails FX2 PON (off, open)  Power rails 1.2 V, 1.5 V and 2.5 V controlled by USB FX2 microcontroller (signal FX2_PS_EN) PS_EN = FX2_PS_EN = 1 or 0 PON (on, closed) Power rails 1.2 V, 1.5 V and 2.5 V always enabled (PS_EN = 1) PS_EN ≠ FX_PS_EN = 1 or 0

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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title Table from EZ-USB(R) Technical Reference Manual (EZ-USB_TRM.pdf).
Table from EZ-USB(R) Technical Reference Manual (EZ-USB_TRM.pdf) Port D Pin Alternate Function Alternate Function is Selected By... Alternate Function is Described in... PD[7:0] FD[15:8] IFCFG1 = 1 and any WORDWIIDE bit = 1 Slave FIFOs chapter 9 on page 99

Signal PS_EN

• Signal PS_EN enables (1) or disables (0) power rails 1.2 V, 1.5 V and 2.5 V.

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  title Power rails 1.2 V, 1.5 V and 2.5 V could be enabled/disabled by signal PS_EN.

  
  

• Power-rail 3.3V is not controlled by signal PS_EN and is unconditionally enabled. 

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  title Power rails 3.3V could not be enabled/disabled by signal PS_EN.

   
  

VCCIO assembly options

Info

 

According to the corresponding assembly option, power rail VCCCIO0

can

(enabled by PS_EN and switch S1B ) can depend or not on the power rail 2.5V.

VCCIO0 voltage can be configured in 3 ways:

• 2.5V - When resistor R103 is populated and resistor R102 is not populated.
• 3.3V - When R103 is not populated and resistor R102 is populated.
• 1.2 V ÷ 3.3 V (External supply) - When R103 is not populated and R102 is not populated. In this case external supply source have to be connected to pins 1, 2, 3, 4 of J4 B2B connector⁽¹⁾.

 ⁽¹⁾ See Spartan-6 documentation fo VCCIO power range.

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title Example of VCCIO0 assembly not dependent on 2.5V power rail. The other way is also possible.
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Warning
Others options of VCCIO0 power supply are not supported and can damage the FPGA!

See the figure below to locate R102 and R103 on PCB.

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title	This figure locates R102 and R103 on PCB.

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See section "FPGA I/O banks power supply" here.

Dip Switch S1B = FX2 PON 

When Dip switch S1B 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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The table below summarizes all switching options implied by Dip switch S1B and firmware signal FX2_PS_EN (under the standard assembly option).