06-04-2019 05:07 PM
I'm a newbie at FPGA and Vivado, and I'm trying to get my outputs to drive 1.8V. I am using the Artix-7. In Vivado under Implemented Design, in the Package Pins window, I have set my pins to LVCMOS18, however when I measure the outputs, I get a voltage level of 3.3V. Is there a way to make the output 1.8V or an I misunderstanding something? Thanks.
06-10-2019 12:09 PM
You need to check which is the power supply to that Bank.
Which Evaluation Board are you using and which pins are you trying to use with LVCMOS18?
06-10-2019 02:05 PM
All the single ended I/O standards drive their outputs "to the rail" - to the voltage applied to the I/O bank of the pin. This is the "VCCO" of the bank. All I/O are part of a bank, which contains a constant number of pins (in the 7 series this is 52 I/O per bank), and all the pins within the bank share the same VCCO.
You are only supposed to use an I/O standard that is compatible with the VCCO of the bank. So using an LVCMOS18 I/O standard in a bank with VCCO=3.3 is "illegal". However, the tools have no mechanism of determining the actual bank voltage, so can't check if the I/O standard you are using is compatible with the actual VCCO voltage on the board - they can merely look for consistency. For example, if you try and place a LVCMOS33 and an LVCMOS18 I/O in the same bank, the tool will issue an error (and not generate a bitstream).
But, presumably, you placed only LVCMOS18 I/O in a bank with VCCO=3.3. The tools did not complain since they did not know about the VCCO, and hence generated a bitstream. The net result is that even though you specified an I/O standard of LVCMOS18, with the VCCO=3.3, the I/O behaves like a 3.3V I/O (with somewhat non-standard characteristics since the I/O is misconfigured).
And, as a direct answer to your question "no" - the I/O levels of an output are determined by the VCCO - regardless of what I/O standard you use you will only be able to get a 3.3V I/O out of a bank with VCCO=1.8. If you need a 1.8V I/O you must use a bank that has a VCCO=1.8V.