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AR# 37474

ISE Simulator (ISim) - Bi-directional port signals appear as undefined "UU"


I have a design that successfully simulates in ModelSim.However, when I use ISim and other 3rd party simulators, I see that bi-directional signals in the design show "UU" instead of the expected result.
How can I resolve this issue?


This issue can occur if there is ambiguity in the output drivers for inout ports. For example, if the following two entities drive each one bit of a two-bit bidirectional bus (data):

architecture arch_foo of foo is // This is instantiated as U0 in top level
data(0) <= '1';
end architecture arch_foo;

architecture arch_bar of bar is // This is instantiated as U1 in top level
data(1) <= '1';
end architecture arch_bar;

it is ambiguous what the drivers for the whole "data" bus are at the scope of each entity.This results in two drivers on each bit of the bus: one assigning a 'U', the other assigning a '1'.
ModelSim ignores this ambiguity and warns the user of the non-adherence to VHDL standards via the following warning:
# ** Warning: (vsim-8684) No drivers exist on inout port /testbench/u0/a(1), and its initial value is not used.
# Therefore, simulation behavior may occur that is not in compliance with
# the VHDL standard as the initial values come from the base signal /testbench/a(1).
# Region: /testbench

To resolve this issue, make sure that all drivers are defined for inout buses. For example:
architecture arch_foo of foo is
a(0) <= '1';
a(1) <= 'Z';
end architecture arch_foo;

architecture arch_bar of bar is
a(1) <= '1';
a(0) <= 'Z';
end architecture arch_bar;
AR# 37474
Date 12/15/2012
Status Active
Type General Article
  • ISE - 10.1
  • ISE Design Suite - 11.1
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