Disable Diode in Cyclone III

[Altera_Forum]

Hi 

 I have a Cyclone III with Vccio=2.5V and want to connect to a DSP with 3.3V Interface. As I understood AN447 I have to disable the diode in Cyclone III. But how can I do this? I read the Quartus Manual, but didn't find anything...  thanks a lot for your help

[Altera_Forum]

The clamping diode is controlled by "PCI I/O" in the Assignment Editor. The Description field for the assignment says this: 

   --- Quote Start ---  Turns on Peripheral Component Interconnect (PCI) compatibility for a pin. For example, when the VCCIO of an EP20K400 device operates at 3.3 V and PCI I/O is turned on for a pin, the Compiler clamps the pin's signal to the VCCIO value, thus making the pin 3.3-V PCI-compliant. This option is ignored if it is applied to anything other than a pin or a top-level design entity.  --- Quote End ---  

[Altera_Forum]

OK, Thank you very much.

[Altera_Forum]

The PCI clamping diodes are disabled by default, so no action is necessary usually.

[Altera_Forum]

I think that's not quite correct Cyclone III Handbook says 

   --- Quote Start ---  The PCI-clamp diode is available for 3.3-V LVTTL, 3.3-V LVCMOS, 3.0-V LVTTL, 3.0-V LVCMOS, PCI, and PCI-X I/O standards. If the input I/O standard is 3.3-V LVTTL, 3.3-V LVCMOS, 3.0-V LVTTL, 3.3-V LVCMOS, PCI, or PCI-X, the PCI clamp diode is enabled by default in the Quartus II software.  --- Quote End ---  But you are right in my case. I have 2.5V

[Altera_Forum]

You are right, the default behavíour was changed with Cyclone III. It may be active even with 2.5V I/O standard.

[Altera_Forum]

Yes, I found a statement in AN447 

  --- Quote Start ---  Notes to Table 3: (1) By default, the Quartus II software enables the PCI-clamp diode on any Cyclone III I/O pin assigned as input, bi-directional, or tri-stated output for 3.3/3.0/2.5-v LVTTL/LVCMOS I/O standard.  --- Quote End ---  So for 2.5V the diode is enabled too...

[Altera_Forum]

In the Fitter compilation report tables like "Input Pins", check the "PCI I/O Enabled" column to know for sure what you actually got regardless of the documentation or attempted assignments.

[Altera_Forum]

Thank you.