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Confused about VBUS Connection from USB Connector | Cypress Semiconductor

Confused about VBUS Connection from USB Connector

Summary: 2 Replies, Latest post by aasi on 19 Jul 2012 09:52 AM PDT
Verified Answers: 0
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Abhey's picture
2 posts


I am using Cypress FX2LP in my interface design with Xilinx Spartan3 FPGA.. Based on Xilinx reference design schematics (from Digilent Nexys-3 Board and Xilinx Spartan3 Evaluation Kit), I found that VBUS from the USB connection may be left floating for self powered designs.

I left VBUS Pin from USB connector as "no connect" and connected wake pin of FX2LP on GPIO of FPGA. On the custom board, I found that FX2LP will not appear in the System Device list unless I pull the WAKE line HIGH from FPGA. Therefore, I pulled the FX2LP wake line high. Since then, I apparently had no problems with my design, except, once in a million times, I have to perform a power cycle as the device won't show in the system devices.

Recently, I came across Cypress AN15813, that states that monitoring the VBUS signal is very important and must be used in conjunction with wake line on FX2LP.

My Question is:

1- Is it mandatory to monitor VBUS line in self powered mode while using FX2LP in Slave FIFO Mode, I did not find such connectivity in XIlinx based Reference Designs?

2- Can FX2LP drive D+/D- lines if wake pin is driven high in Slave FIFO Mode, while nothing is driven by the FPGA controller on the data lines, and WAKE pin is pulled up? 




prji's picture
Cypress Employee
473 posts


    A USB-IF specification requirement is to never drive the USB pins when they are not connected to the bus. In order to meet this spec requirement it is always recommended to use a Vbus monitor circuit in self-powered devices. 

   Please go through 6.3 Wakeup/Resume of TRM for more information on wakeup pin and its usage.



aasi's picture
Cypress Employee
1166 posts

 Please look at for complete detail on why VBUS monitoring is needed.

As to the use of WAKE pin to make the chip enumerate in your scenario. It might have to do with hardware issues. Does your crystal meet the parameters specified in FX2LP datasheet?

When you run into the non-enumeration scenario is there oscillations on the crystal pins?



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