AR# 7843

Description

General Description:

What resistor value should I use to externally pull the outputs to 5V?

Solution

Pulling the outputs to 5V is only allowed with the following three I/O standards: LVTTL, LVCMOS, and PCI33_5. Note that outputs that are pulled up should be put into a 3-state condition.

The internal I/O pull-ups can be considered 50k ohm resistors to VCCO (3.3V or 2.5V). Consequently, if you want to pull an I/O to a voltage higher than VCCO, you must do this with a resistor smaller than 50k ohms. In Virtex devices, circuitry allows the disabling of the internal pull-up if the I/O is pulled higher than about VCCO + 0.7V (a threshold voltage higher than VCCO). Consequently, if you pull an I/O up to 5V, there is no static current into VCOO. However, if you do not pull up an I/O past VCCO + 0.7V, the internal pull-up remains enabled and "fights" with whatever is trying to pull up the I/O.

Xilinx recommends using a 4.7k ohm pull-up, but any value less than this is effective.

For more information on the 5V interface in Virtex devices, reference:

http://www.xilinx.com/products/virtex/techtopic/vtt002.pdf