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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 helpLink Copied
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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 ---- Mark as New
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OK, Thank you very much.
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The PCI clamping diodes are disabled by default, so no action is necessary usually.
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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- Mark as New
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You are right, the default behavíour was changed with Cyclone III. It may be active even with 2.5V I/O standard.
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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...- Mark as New
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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.
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Thank you.
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