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AR# 39234

Spartan-6 - Vccaux - When should a 3.3V or 2.5V Vccaux voltage be used?


The Spartan-6 FPGA allows a Vccaux voltage of 2.5V or 3.3V. What factorsshould be considered in choosingtheVccaux voltage?


The main factor for picking one over the other is the supply voltages available on the board. Having the choice between 2.5V and 3.3V means that there is greater flexibility and the potential to save a supply source on the board.

From a design point of view the biggest difference is the I/O standard compatibility. It will dependon the use case which Vccaux level suits the best. For example, to use LVCMOS25 inputs in a non-2.5V bank Vccaux has to be set to 2.5V. The Spartan-6 FPGA SelectIO User Guide, UG381,contains further information on this. Anotherpoint of note from the Spartan-6 FPGA Configuration Guide, UG380, is that if VCCO_2 is 1.8V, VCCAUX must be 2.5V. If VCCO_2 is 2.5V, VCCAUX can be either 2.5V or 3.3V.

The other minor differences are the tolerance of the DIFF_TERM termination and the power consumption.
The DIFF_TERM termination is powered by Vccaux. The tolerance varies with Vccaux as well.
Taken from the IBIS File:
typmin max
Vccaux = 2.5V: 100175 80
Vccaux = 3.3V: 100 120 80

The power consumption will be less for a Vccaux of 2.5V. TheSpartan-6 FPGA XPE spreadsheetcan be used tocompare the power consumption of both Voltage supplies.
AR# 39234
Date 12/15/2012
Status Active
Type General Article
  • Spartan-6 LX
  • Spartan-6 LXT
  • Spartan-6Q