Nexys-2 FPGA board and design issue

[Maxwell077]

Dear All,

I'm doing a project based on Nexys-2 FPGA board.
A sensor module and FT2232H USB module are connected to Pmod connectors of Nexys-2.

There are two modules in my FPGA design, one is sensor interface module, other is FT2232 module.
Sensor interface samples output of my sensor, and routes it to FT2232 module.
FT2232 module captures data and sends it to PC over USB.

While debugging my design with Chipscope, I have faced with an interesting issue.
While board is powered over USB port, I am observing my internal signals(FPGA internal signals - sensor module and FT2232H module port mapping signals, not output or input ports) and assume that have a value 0x55;

While board is powered with adapter, same internal signal value is 0x90 etc - something impossible. I mean, internal signal value is changed. However, it should be same. I also changed the jumpers.

Powering with adapter should give correct results however it is not. Because, sensor needs 400mA and it is dangerous to connect the board to USB.

How could it be? Any ideas, I freak out. 8-O

 

[mrflibble]

About powering the nexys2 over usb ... I once spent a nice amount of quality time debugging a design. Couldn't figure out why the hell the thing was not working properly when I used LVCMOS33 io standard. Things worked perfectly on 2.5 volt, just not on 3.3 volt. Only to find out that when all my problems magically disappeared when I used a proper 5 Volt supply instead of the usb power.

So basically, when in doubt: use a proper 5 volt supply. And set your jumpers correctly obviously. ;-)


In my experience the nexys2 is less problematic when you use the power adapter, so when you you are getting more problems with the adapter then personally I'd suspect this adapter. So ... when powering from an adapter, have you checked the voltages on the PMOD connectors? As in do you get your proper IO voltage you are expecting?

 

[Maxwell077]

Thanks mrflibble,
I'm using Spartan 3E Starter board adapter. As you said, I should check voltages from Pmod connectors.
But, it is really exciting problem.