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Adventurer
Adventurer
8,196 Views
Registered: ‎11-10-2009

Output port with default value ('Z') showing wrong electrical level

Hello all,

 

in the top level of my design I have described a port in the following way:

 

port : OUT std_logic := 'Z';

 

The logic which will drive this port in the future is not yet existent so I could think that the FPGA would show a high impedance output at that pin. Unfortunately I can see that the FPGA drives that pin to a clear '0'.

 

I have an external pull-up resistor in the line driven by that pin, which shows a correct '1' when the FPGA is not programmed.

 

First I thought that this could be due to the fact that my -g UnusedPin option is "Pull down" and, lacking any logic behind the pin more than the default 'Z' value, the ISE could consider that pin as unused IOB. I set this option to "float" but no difference could be appreciated.

 

It will also work if I define the port as:

 

port : OUT std_logic := '1';

 

which, by the way, is not legal for my hardware.

 

Can anybody give me a clue about this strange phenomen?

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