What does Vtt in the XPower report mean?
When I run XPower and I have an IO with a GTL standard in my design, I see a Vtt parameter reported in XPower. What is Vtt? Where does it come from?
Vtt refers to voltage associated with any off-chip power supplies that source current which is sunk on-chip. Vtt values are reported only with open drain I/O conditions (driver drives a logical "0" or floats) especially when I/O standards like GTL, GTLP and I2C are used.
Drivers configured with these standards, when driving a zero, actually sink current from an off-chip source. It does affect the Xilinx device, so we have added it in the calculations; from a thermal analysis standpoint, XPower accounts for this heat.