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Terminations
Table 2:
Unused I/O Termination Schemes
R
Unused I/O Termination Schemes
All Pulled-up (not possible if Bus-hold is selected
for input termination)
All Bus-hold (not possible if Pull-up is selected for
input termination)
All Configurable Ground
All Float
It is not recommended to float a CPLD I/O, as it consumes extra current than if held in a
known state. You should select float as a termination mode only if the pin has external
terminations.
Setting Termination Modes in Software
The termination modes can either be selected globally, as CPLDFit options, or on a
pin-by-pin basis using constraints.
If you are using the ISE tools GUI:
1.
2.
3.
Select the top level in the Sources window.
In the Processes window, select CPLDFit.
Right-click and select Properties. The Fit Properties dialog box appears.
There is a drop-down menu for the Input and Tristate I/O termination mode. For the
CoolRunner-II and CoolRunner XPLA3, there is a drop-down menu for the Unused I/O
Pad termination. However, if the design targets an XC9500, XC9500XL, or XC9500XV, there
are only two options for the unused termination; by default, this is float. The user has the
option to Create Programmable Ground pins on Unused I/Os; if this check box is ticked,
the pin can be considered as outputs driving low.
To constrain the termination on a pin-by-pin basis, use the following UCF example:
NET "My_NET" KEEPER;
For more information on the constraints syntax, see the Constraints Guide in the software
manuals, found at:
www.xilinx.com/support/software_manuals.htm .
Termination Mode for Outputs
It is not possible or necessary to select a termination mode for output pins. The output of a
macro cell, which drives the output, is always at logic 1 or logic 0. Therefore, there is no
need to terminate the output. Outputs that are in a Tristate condition can be terminated.
Bus-hold/Keeper
The bus-hold or keeper is present in the CoolRunner-II, XC9500, XC9500XL, and
XC9500XV families. It is internal circuitry that drives the last known value on the I/O into
the device. The bus-hold circuit is only on when the device is in user mode and keeper
termination mode is selected. Therefore, if the device is unconfigured or in the In-System-
10
CPLD I/O User Guide
UG445 (v1.2) January 14, 2014
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