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AR# 19811

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


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?


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:


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


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


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


AR# 19811
Date 12/15/2012
Status Active
Type General Article
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