Answers Database
Virtex: How to pull the IOs to 5V with external Pullups
Record #7843
Product Family: Hardware
Product Line: Virtex
Product Part: Virtex General Hardware
Problem Title:
Virtex: How to pull the IOs to 5V with external Pullups
Problem Description:
Urgency: Standard
Problem Description:
What resistor values should one use to pull the IOs to 5V externally
Solution 1:
To pull the IOs to be 5V externally, only 3 IO standards are allowed:
LVTTL, LVCMOS, and PCI33_5.
The internal IO pullups can be considered to be about 50K resistor to
Vcco(3.3V or 2.5V). If you try to pull an IO to a voltage higher than Vcco,
you need to do so with a resistor with a smaller value than that. There is
circuitry to disable the internal pullup if the I/O is pulled higher than about
Vcco + 0.7V (a threshold voltage higher than Vcco), so if you pull an I/O
up to 5V, there will not be static current into Vcco. But if you can't pull
an I/O up past Vcco + 0.7V, then the internal pullup remains enabled
and will "fight" with whatever is trying to pull the I/O up.
Our recommendation is to use a 4.7K ohm, but anything less than that is fine
End of Record #7843 - Last Modified: 10/11/99 13:52 |
