Change the input voltage of the FPGA

[HyperText]

Hi,

I'm trying to change the input voltage of a Xilinx Spartan-3E FPGA (the default one is 1.2V). The board is a Digilent Nexys 2, and of course a tension regulator is involved, hence I have to consider it.
The schematic is the following:

SW1 is the output of the tension regulator and its value is of 0.8V. VCC1V2 is set using the formula: VCC1V2 = 0.8V * [(R78+R79)/R79].
So, by changing the R78 value I can change the input voltage of the FPGA.

However, a user in the Xilinx Forum suggested another solution (https://forums.xilinx.com/t5/Xilinx-Boards-and-Kits/Nexys-2-board-variable-supply-to-core/m-p/116380/highlight/true#M3707) consisting of: "Lift one side or the other of L2. You can connect a variable output bench supply directly to the VCC1V2 net.".
Since I'm not an Electronic, I would like some additional informations:

• I just lift one side of L2 and connect a different power supply (let's say 0.9V, coming from a dedicated external power supply), without any particular precaution?
• how should I calculate the VCC1V2 value? Using the above 0.9V value, VCC1V2 will still follow the above equation? So, VCC1V2 would be equal to: 0.9V * [(R78+R79)/R79]?

Thanks for any hint,
Pierpaolo

 

[Wade Hassler]

Adding an external supply would work. however, there may be sequencing rules on which of the several power-supplies must be applied first.
Replacing or modifying R78 is a much better way. If R78 were parallelled with another, its value would go down and the voltage at VFB1 would go up: what you want.
I urge you to find somebody with a soldering iron and steady hands to parallel R78 with (I think) a 70.6K resistor
(You should also find someone who can commit to that number)

 

[kubeek]

I am not sure why you call that the input voltage, when (using the datasheet terminology) it is most likely the Internal supply voltage VccInt.
Also I would like to know why you want to go to 0.8V when the recommended operating voltage is 1.14 to 1.26V for VccInt.
1.1Vmin is for I/O supply and 2.375 for Vccaux

 

[RichTheDude]

Ditto kubeek. Generally the core voltage is 1.2V and each bank has it's own VCCIO pin, which can be in the range of 1V to 3.3V give or take a bit. To change the IO voltage, change VCCIO not the core voltage or use a translator IC if that is not possible.

The core should always be at 1.2V or the parts specified voltage.

 

[koolguy]

Hi HyperT,
Do you work in FPGA?

 

[Mikebits]

You don't need to change Jack squat on your hardware. All IO voltages are programmable. Just set the IOSTANDARD Attribute for the voltage you are using (see appropriate datasheet). It has been a long time since I used Xilinx FPGA, but your development tool should have a constraint editor.

 

[kubeek]

Mike, first look at how old the thread is, and then look which well-known member decided to put his two cents in.
OT: Shouldn´t there be some kind of moderation in place when people are replying to old threads?

 

[Mikebits]

I just looked at the last post date, guess I need to pay more attention