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AR# 21034

Spartan-3, DCI/XCITE - Not all DCI inputs or outputs require reference resistors on VRN and VRP. Can I use VRN/VRP as regular I/O when DCI is not used?

Description

The following DCI outputs do not require reference resistors on VRN and VRP:

HSTL_I_DCI

HSTL_III_DCI

HSTL_I_DCI_18

HSTL_III_DCI_18

SSTL2_I_DCI

NOTE: There is no standard available for LVDS DCI output I/O. See Note 1 in the resolution section for more information.

The following inputs do not require reference resistors on VRP and VRN:

LVDCI_15

LVDCI_18

LVDCI_25

LVDCI_33

LVDCI_DV2_15

LVDCI_DV2_18

LVDCI_DV2_25

LVDCI_DV2_33

There is no termination on these inputs. An LVDCI_25 input is the same as a LVCMOS25 input.

Does this mean that VRN and VRP can be used as general I/O in a bank that contains these outputs?

Solution

Yes. For the inputs and outputs listed above, DCI termination is not used. Consequently, you can use the VRN/VRP as regular I/O. These input and output buffers are available as naming conventions only to match them with their respective outputs and inputs.

If you choose to use VRN and VRP as general I/O, make sure that the DCI I/O standards listed above are the only DCI standards (input or output) in the appropriate bank.

NOTES:

1. More information is available in the Spartan-3 Data sheet located at:

http://www.xilinx.com/xlnx/xweb/xil_publications_display.jsp?sGlobalNavPick=&sSecondaryNavPick=&category=-1209725&iLanguageID=1
2. The 25 ohm resistor in series with the SSTL2_I_DCI outputs is not DCI-controlled.

3. You should always perform IBIS simulation to verify the signal integrity on any PCB trace. You can download Xilinx IBIS models at:

http://support.xilinx.com/support/sw_ibis.htm

AR# 21034
Date Created 09/04/2007
Last Updated 12/15/2012
Status Active
Type General Article