Answers Database

XEMAKE/XEMAKE6: schematic design functions incorrectly in simulation or in system


Record #667

Product Family:  Software

Product Line:  EPLD Core

Problem Title:
XEMAKE/XEMAKE6: schematic design functions incorrectly in simulation or
in system



Problem Description:
When using the 5.2 or 6.0 software, a design may not function correctly during
simulation or in the target system.


Solution 1:

If the following warning is issued during the fitting process:

dr25:[Warning]Hanging input (<instance_name>:<pin>)

then the problem may be related to unconnected macro pins on the
schematic.

The XEPLD 5.2/6.0 software does not tie unconnected pins to VCC or GND,
nor does it stop with an error if unconnected CE or LOAD pins are encountered.
It just gives a warning that the pin is unconnected and may create incorrect
logic.

Library macros with mode pins (like the CE of a flip flop or the LOAD of a
counter) are implemented with muxes in front of flip flops.  The mode pins are
the select lines on the mux.  If one of the select lines of the MUX is left
unconnected then the mux is reduced to an OR gate.  This logic is usually
incorrect.

Macro input pins should not be left unconnected in XC7000 schematics,
especially LOAD or CE pins.  If these pins are not used in the design, they
should be connected to VCC or GND on the schematic.



End of Record #667