Xilinx UCF: Adding "Virtual Grounds"

i.e. via a constraint in the UCF file

I thought someone might have replied by now.

The only method I can think of is to put a wrapper around your VHDL code, with the extra ground pins tied to GND, and to synthesise that.

At least in earlier versions, I did stuff like that with a separate UCF line for each thing I was trying to do. So, I didn't try to put a LOC and a PULLUP or PULLDOWN on the same line. But, the ground function is a CPLD feature that is not present on FPGAs, isn't that right?

Jon

i.e. via a constraint in the UCF file

You can't infer drivers from the .ucf file. If you want the gounded pins to act as "virtual grounds" to reduce ground bounce, then you need to add them to your design and assign them to zero.

If you really can't change the VHDL for some reason, then you can achieve the same thing using the FPGA editor, but that method is more painful.

Just out of curiosity, why don't you want to change the VHDL code?

-- Gabor

VHDL i.e. via a constraint in the UCF file

LVCMOS25;

LVCMOS25;

IOSTANDARD = LVCMOS25;

IOSTANDARD = LVCMOS25;

IOSTANDARD = LVCMOS25;

Hi All, I am using an FPGA evaluation board to prototype and ASIC, and I want to avoid (or at least minimize) changes to the RTL between ASIC and FPGA.

I was assuming that you could achieve what I wanted via UCF, but it looks like I was wrong :-(

Anyway, updates to VHDL it is.

Thanks, Steven

VHDL i.e. via a constraint in the UCF file

LVCMOS25;

LVCMOS25;

IOSTANDARD = LVCMOS25;

IOSTANDARD = LVCMOS25;

IOSTANDARD = LVCMOS25;

:-) )

(or at least minimize) changes to the RTL between ASIC and FPGA.

I was wrong :-(

Probably the easiest way to avoid changes from the ASIC version is to make a wrapper for the FPGA version that mostly instantiates and hooks up the existing ASIC design, and adds any FPGA-only infrastructure like virtual grounds, internal power-on resets, etc.

If I were doing this in Verilog, I'd probably use the preprocessor to ifdef the FPGA additions, but the wrapper works in any HDL.

-- Gabor