What this error means
This is the input-pin specialist of the missing-unit family. After ERC
finds that a reference has fewer placed units than its library symbol
defines, it opens the library definition of each unplaced unit and
sorts its pins by electrical type. Any Input pin puts that unit on
this warning's list; power inputs and bidirectional pins go to
missing_power_pin and missing_bidi_pin instead. One warning per
reference:
Symbol U1 has input pins in units [ B ] that are not placed (KiCad 9
prints a trailing period, 10 does not).
The pins it names sit on no net, so no connectivity check will ever see them. On the assembled board they are real, and a floating CMOS input oscillates and burns power.
Common causes
1. A spare section was left off the schematic
The second amplifier of a dual op-amp, the unused gates of a quad
package: you drew the sections you needed and stopped. The skipped
sections have input pins, so beyond the umbrella missing_unit
warning you get this one, flagging inputs that will float unless
the board handles them.
2. A rename or reannotation split the package
Units are grouped by reference designator and nothing else. Edit one section's reference by hand (U1 to AR1), or reannotate so a section lands in a different number, and each resulting group is missing the units the other holds — while every section is plainly visible and wired on the sheet.
How to fix it
Place the missing unit and tie its inputs off
- Press A (Add Symbol) and place another copy of the symbol.
- Right-click it → Symbol Unit → the unit the warning names, and set its Reference to the existing package.
- Wire each input to GND or the supply, per the datasheet's unused-section advice, and put a no-connect flag (Q) on the output.
- Re-run ERC (Inspect → Electrical Rules Checker). Menu paths are the same in KiCad 9 and 10.
Use when: The unit is genuinely absent — the normal case. Tying the inputs is the datasheet's handling for an unused CMOS section, not ERC housekeeping.
Reunite a split reference
- Compare the reference on every section of the flagged symbol: one carries a different number.
- Double-click the stray section and set its Reference back to the shared package.
- If a reannotation scrambled things sheet-wide, run Tools → Annotate Schematic… and re-run ERC.
Use when: Every section is on the schematic and wired, yet ERC insists units are missing — the package is split across two references.
When it’s safe to exclude
Treat this one more cautiously than its umbrella sibling. Excluding
missing_unit records "I know a section is unplaced"; excluding this
warning hides the consequence — input pins that will float on the
finished board, which no other check will mention.
Excluding (right-click the marker → Exclude) is defensible when the datasheet says the unused section needs no input termination, or your team terminates spares in layout as a matter of policy. Otherwise placing the unit and tying two pins costs less than the debugging a floating input can cause. Downgrading the whole check in Schematic Setup → Electrical Rules → Violation Severity also silences the split-reference case, which is a genuine mistake.
Gotchas
- The family is skipped when the number of placed instances equals the library's unit count — instances, not distinct units. Two copies of unit A on a two-unit part settle the headcount; the all-input unit B is never mentioned (verified, KiCad 10.0.7).
- Placing the unit converts this warning into
pin_not_connectederrors until the inputs are tied off. That is progress: ERC can finally track them.
Minimal reproduction
Place one unit of a symbol whose other units have input pins — one
amplifier of a dual op-amp will do — annotate it, and run ERC.
Alongside the umbrella missing_unit row you get:
[missing_input_pin]: Symbol U1 has input pins in units [ B ] that are not placed
; warning
The checked-in file data/erc/samples/missing_input_pin.kicad_sch
reproduces this with one embedded two-unit symbol: unit A (an output
and a passive pin, wired) placed, unit B (two input pins) never
placed. KiCad 9.0.9 and 10.0.7 both report the line above — 9 with a
trailing period — plus the missing_unit row and one
lib_symbol_issues warning about the missing repro library, an
artifact of the self-contained sample. Placing unit B under U1 with
its pins wired clears every missing-unit row.
Related errors
- Symbol has units that are not placed
- Symbol has power input pins that are not placed
- Symbol has bidirectional pins that are not placed
- Symbol has more units than are defined (reference page coming soon)
FAQ
All terminals of my op-amp are connected — why does ERC report input pins in units that are not placed?
The pins it means are not on your schematic: whole units are missing under that reference, and those units contain input pins. Check the reference on each section — a hand-renamed or reannotated section (U1 turned into AR1) splits the package so each reference is missing the units the other holds.
What should I do with the inputs of a gate I'm not using?
Tie them to ground or the supply rail, following the datasheet's unused-input note, and no-connect the output. Floating CMOS inputs oscillate and raise supply current; this is board hygiene, not ERC appeasement.
Can I put no-connect flags on the spare gate's inputs instead of wiring them?
A no-connect flag silences ERC, but the physical input still floats, and CMOS inputs are not designed to. Reserve no-connect for the spare section's output and wire the inputs to a rail.