XEM6001 Self Powered Operation

Spartan-6 USB 2.0 Modules
1

I am trying to use an XEM6001 as a self powered device by removing R26.

The XEM6001 does power up and 3.3V is present on JP4, Front Panel does see the board but it will not load a configuration.

If I short the R26 pads while the external 3.3V is still applied Front Panel will load the configuration and the FPGA starts up. When I remove the short across the R26 pads while the external 3.3V is still applied the FPGA appears to loose its’ configuration.

The external 3.3V is rock solid.

Any ideas ?

2

We did a quick experiment here to confirm correct operation and did not encounter any issues. We removed R26 and powered the 3.3V bus via JP1. Configuration via FrontPanel USB as well as boot from PROM worked as expected. Our first suspicion would be the external power. Is the supply providing enough current? How is the external supply connected? Are there any potential ground loops via other attached circuitry?