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AR# 20663

Spartan-3 Configuration - Why do the mode pins have a voltage on them when using external pull-down resistors?

Description

I have the mode pins tied to ground using external pull-down resistors. Why do the pins have a (non-zero) voltage?

Solution

Spartan-3 devices have an internal pull-up on the mode pins to make slave serial the default mode if the pins are left unconnected. These internal pull-ups are stronger than previous families (see (Xilinx Answer 19024) to calculate the effective resistance).

The strength of the internal pull-up and pull-down resistors is defined by the parameters Irpu and Irpd in the Spartan-3 Data Sheet, found at:

http://www.xilinx.com/xlnx/xweb/xil_publications_index.jsp

To ensure that the mode pin senses a low (or logic '0'), the voltage must be below the maximum value for the Vil (where VIL is the input voltage that indicates a Low logic level).

The mode pins are LVCMOS25. From the Spartan-3 Data Sheet, Vil max for the LVCMOS25 standard is 0.7V.

Therefore, the external resistor (Rext) must be Rext < (0.7 V/ Irpu) where Irpu is the current through the pull-up resistor.

For example, if Irpu is 1.41 mA, then Rext must be less than 496 ohm (for example, 470 ohm).

AR# 20663
Date Created 09/03/2007
Last Updated 12/15/2012
Status Active
Type General Article