Forum Discussion
FBels
New Contributor
New ContributorWhat is the state of a Cyclone IV unconnected input that goes to user logic
Say a user design consists of an XOR gate. Input A, Input B, Output C. Always @(*) C = A && ~B || ~A && B; For whatever reason though the PCB does not connect input A to anything. W...
FBelsNew Contributor
New Contributorak6dn et al,
I'm also concerned that stray low-current HV ESD impulse noise can fry an unconnected non-pulled up or non-bus-hold input, assuming it has negligible ESD withstand capability. Concern that a fairly low current ESD/surge stray can furthermore induce catastrophic latch-up .
What is even more concerning is what to do with a Cyclone IV's unconnected dedicated clock input pins! As far as I know there is no option for enabling bus hold or weak pullup in their assignments.
ak6dn
Regular Contributor
Regular ContributorFor any unused input, enable the passive pullup. No reasonable designer would let a CMOS input float. Don't do it.
Enable passive pullup, or add an external pullup/pulldown, as appropriate, for any iinput that may ever possibly not be driven by an external source.
