7 Series FPGAs SelectIO Resources User Guide www.xilinx.com 49
UG471 (v1.10) May 8, 2018
7 Series FPGA SelectIO Attributes/Constraints
The DRIVE attribute uses the following syntax in the UCF file:
INST <I/O_BUFFER_INSTANTIATION_NAME> DRIVE = "<DRIVE_VALUE>";
PULLUP/PULLDOWN/KEEPER Attribute for IBUF, OBUFT, and IOBUF
Input buffers (e.g., IBUF), 3-state output (e.g., OBUFT), and bidirectional (e.g., IOBUF)
buffers can have a weak pull-up resistor, a weak pull-down resistor, or a weak “keeper”
circuit. This feature can be invoked by adding the following possible constraint values to
the relevant net of the buffers:
• PULLUP
• PULLDOWN
•KEEPER
The logic level can be set globally for unused I/O using the
BITSTREAM.CONFIG.UNUSEDPIN property
. For more information on implementing these
attributes on either individual I/Os or globally for all I/Os, see the pull-up, pull-down,
and keeper descriptions in UG625
: Constraints Guide (for the ISE tools) or UG912: Vivado
Design Suite Properties Reference Guide. For more information on implementing device
configuration bitstream settings, refer to UG628
: Command Line Tools User Guide (for the ISE
tools) or UG908
: Vivado Design Suite User Guide: Programming and Debugging.
Differential Termination Attribute
The differential termination (DIFF_TERM) attribute supports the differential I/O
standards when used as inputs. It is used to turn the built-in, 100Ω, differential termination
on or off. The on-chip input differential termination in 7 series devices provides major
advantages over using a discrete resistor by removing the stub at the receiver completely
and therefore greatly improving signal integrity. Additionally it:
• Consumes less power than DCI termination
•Does not use VRP/VRN pins (DCI)
This attribute can be applied to input pins for the following I/O standards:
•LVDS
•LVDS_25
• MINI_LVDS_25
• PPDS_25
•RSDS_25
The V
CCO
of the I/O bank must be connected to 1.8V for LVDS, and 2.5V for the other
differential I/O standards to provide 100Ω of effective differential termination.
DIFF_TERM is only available for inputs and can only be used the appropriate V
CCO
voltage.
The DIFF_TERM attribute can be specified in the UCF constraints file or by setting the
appropriate value in the generic map (VHDL) or in-line parameter (Verilog) of the
instantiated IBUFDS, IBUFGDS, IBUFDS_DIFF_OUT, or IOBUFDS_DIFF_OUT primitives.
Refer to the ISE tools language templates or the 7 series FPGA HDL Libraries Guide for the
proper syntax for instantiating these primitives and setting the DIFF_TERM attribute.
The allowed values for the DIFF_TERM attribute are:
•DIFF_TERM=TRUE
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