I am working on a Cyclone V design that is utilizing most of the pin resources. In my first pass, I let the fitter automatically assign pins.The issue that kept coming up is that, if not mitigated, the fitter would assign 2.5V outputs and 3.3V inputs to the same I/O bank. No errors or warnings are issued and the design compiles successfully with this configuration. To my knowledge, this is not valid. Unless I'm misinterpreting the rules, 3.3V outputs are compatible with 2.5V inputs (with VCCIO=VCCPD=3.3V), but 2.5V inputs are NOT compatible with 3.3V inputs (regardless of VCCIO/VCCPD setting). Am I reading this document correctly (Table 5-10): http://www.altera.com/literature/hb/cyclone-v/cv_52005.pdf
Hi RC.Looking at your document, I agree with you. Looking at this table, it appears VCCIO of 2.5 support 3.0 volt inputs. They may have some floating reference voltage in the IO structure that allows this however. Looking at the datasheet http://www.altera.com/literature/hb/cyclone-v/cv_51002.pdf Table 14 shows that for the 2.5V vccio it supports a VIH max of 3.6 V. This would allow 3.3 input operation. (with the recommended clamping diodes) So my guess is the table 5-10 is in error, and they really support 3.3V inputs on a 2.5V bank. However, I would contact altera support to confirm this. In either case, I would still recommend you separate the 3.3V bank form the 2.5V bank, just to avoid personal confusion going forward, unless you have ran out of IOs. Pete
Pete,Thanks! Unfortunately, the latter scenario is the case, and while I've minimized the occurrence of mixed voltage I/O on a single bank, I don't have enough room to eliminate it completely. I actually don't think the datasheet is in error. As you mentioned, with VCCIO=2.5V it can take up to 3.6V, but it is not preferred practice. I have conformed to Table 5-10 even though Quartus doesn't enforce it. I will check with Altera, and post back on this. Thanks again. Bob
I think you should refer to the Cyclone V datasheet, Table 14. Single-Ended I/O Standards for Cyclone V Devices (Part 1 of 2)It specifies a Vih range of 1.7 to 3.6 V for 2.5V IO-standard.
--- Quote Start --- I think you should refer to the Cyclone V datasheet, Table 14. Single-Ended I/O Standards for Cyclone V Devices (Part 1 of 2) It specifies a Vih range of 1.7 to 3.6 V for 2.5V IO-standard. --- Quote End --- Thanks, FvM. Seems like the datasheets are suggesting more flexibility than I originally thought.