UPGRADE YOUR BROWSER

We have detected your current browser version is not the latest one. Xilinx.com uses the latest web technologies to bring you the best online experience possible. Please upgrade to a Xilinx.com supported browser:Chrome, Firefox, Internet Explorer 11, Safari. Thank you!

AR# 19811

Spartan-3/-3E - Is VCCO required on a bank that has differential or VREF-dependent inputs?

Description

Spartan-3/-3E differential and VREF-dependent inputs (LVDS, LVPECL, SSTL, HSTL, GTL, and GTLP) are powered by VCCAUX. Is a VCCO supply still required for a bank that houses these inputs?

Solution

Yes, a VCCO supply is still required.

The input clamp diodes become forward-biased if VIN exceeds VCCO by the diode threshold voltage (~0.5V). This results in leaving VCCO disconnected or setting it to an excessively low voltage that can lead to a distorted input signal.

To avoid this, VCCO should be greater than the input high voltage (plus any overshoot), less the 0.5V diode voltage. For example, if the maximum input high voltage is 2.5V (including overshoot), VCCO should be greater than 2.5V - 0.5V = 2.0V.

NOTE: The clamp diodes in Spartan-3/-3E are not I/O standard-dependent; they are always present.

A VCCO supply is always required to achieve proper termination on Spartan-3 DCI inputs.

For more information, see the functional description data sheet.

The Spartan-3 Functional Description Data Sheet is located at:

http://www.xilinx.com/xlnx/xweb/xil_publications_display.jsp?sGlobalNavPick=&sSecondaryNavPick=&category=-1209725&iLanguageID=1

The Spartan-3E Functional Description Data Sheet is located at:

http://www.xilinx.com/xlnx/xweb/xil_publications_display.jsp?sGlobalNavPick=&sSecondaryNavPick=&category=-1211389&iLanguageID=1

Other relevant references include the Spartan-3 Generation FPGA User Guide (UG331):

http://www.xilinx.com/support/documentation/user_guides/ug331.pdf

and the Spartan-3 Generation Configuration User Guide (UG332):

http://www.xilinx.com/support/documentation/user_guides/ug332.pdf

AR# 19811
Date Created 09/03/2007
Last Updated 12/15/2012
Status Active
Type General Article