Honored Contributor
Honored ContributorAh, thanks for the reassurance and suggestions!
Adding an external 12Kohm pulldown resistor to any user IO pins makes it hover at around 0.375V which is down from 1.641V; even though VCCIO is at a solid 1.8V. Solving for the unknown resistance in the device yields an internal pull up of 40Kohm. According to Table 1-12 on page 1-11 of the Cyclone 4 handbook that appears to be close to the nominal internal weak pullup resistance value. Interestingly enough, if I set the same VCCIO banks to 3.3V, the user IO pins hover at around 3.111V and go down to 1.41V when I add the 12Kohm pulldown. This yields 14.4K for the internal pullup resistance, which is on the low side but still acceptable for R_PU at VCCIO=3.3V. It seems almost as if every user IO is being powered by internal weak pull-up resistors. The device does this as soon as it is booted and without any configuration loaded. Lastly, there are a few LEDs attached to the C4 that light up very brightly when a configuration that tells them to light up is loaded. However, before the device is configure, the LEDs light up just barely. Every VCCIO is set to 3.3V for the entire duration of the test. This again makes it seem like the internal pullups are powering the device at reset. Quick edit: the device and all of its user IO pins seem to work perfectly once the device is configured.
