Honored ContributorHi
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
Honored ContributorThe 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 ---Honored ContributorOK, Thank you very much.
Honored ContributorThe PCI clamping diodes are disabled by default, so no action is necessary usually.
Honored ContributorI 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 2.5VHonored ContributorYou are right, the default behavíour was changed with Cyclone III. It may be active even with 2.5V I/O standard.
Honored ContributorYes, 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...Honored ContributorIn 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.
Honored ContributorThank you.
