Xilinx Constraints Guide

IOSTANDARD (Input Output Standard)
The IOSTANDARD (Input Output Standard) constraint:
• Is a basic mapping constraint.
• Is a synthesis constraint.
IOSTANDARD for FPGA Devices
Use IOSTANDARD to assign an I/O standard to an I/O primitive.
Chapter 4: Xilinx Constraints
All components with IOSTANDARD must follow the same placement rules (banking
rules) as the SelectIO components. See the Xilinx® Libraries Guides for information on
the banking rules for each architecture. For descriptions of the supported I/O standards,
see the device data sheet.
For Spartan®-3, Spartan-3A, Spartan-3E, Virtex®-4, and Virtex-5 devices, the
recommended procedure is to attach IOSTANDARD to a buffer component instead
of using the SelectIO variants of a component. For example, use an IBUF with the
IOSTANDARD=HSTL_III constraint instead of the IBUF_HSTL_III component.
For Spartan-3, Spartan-3A, Spartan-3E, Virtex-4, and Virtex-5 devices, differential
signaling standards apply to IBUFDS, IBUFGDS, OBUFDS, and OBUFTDS only (not
IBUF or OBUF).
IOSTANDARD for CPLD Devices
Architecture Support
Applicable Elements
Propagation Rules
You can apply IOSTANDARD to I/O pads of CoolRunner-II devices to specify both
input threshold and output VCCIO voltage. For supported values, see the device data
sheet.
The CPLD fitter automatically groups outputs with compatible IOSTANDARD settings
into the same bank when no location constraints are specified.
Applies to all FPGA devices and CoolRunner-II CPLD devices.
To see which design elements can be used with which device families, see the Libraries
Guides. For more information, see the device data sheet.
• IBUF, IBUFG, OBUF, OBUFT
• IBUFDS, IBUFGDS, OBUFDS, OBUFTDS
• Output Voltage Banks
It is illegal to attach IOSTANDARD to a net or signal except when the signal or net is
connected to a pad. In this case, IOSTANDARD is treated as attached to an IOB instance
(IBUF, OBUF, IOB FF). When attached to a design element, IOSTANDARD propagates
to all applicable elements in the hierarchy within the design element.
Syntax Examples
The following examples show how to use this constraint with particular tools or
methods. If a tool or method is not listed, you cannot use this constraint with it.
Constraints Guide
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