Fast multi-key presses on a keyboard with weak anti-ghosting drop some keys

What it is

Cheap keyboards scan keys on a row-column matrix. If three keys happen to sit on a rectangle in that matrix, the controller may read a fourth key that was not pressed (ghosting), or, to avoid ghosts, refuse to report one of the real keys (blocking / dropped keys). Fast chords while typing, or WASD plus a fire key in a game, lose that unreported key. It is scan geometry, not a finger that missed, and not software dropping the event.

Why it happens

The matrix drives rows and reads columns. Three closed switches on two rows and two columns let current sneak through the fourth crossing, so the controller thinks the fourth key is down. Firmware often then stops reporting new keys on a dangerous combination, so the user pressed and no down event appears. Diodes block the sneak path, at cost and board-layout expense. Office membranes often guarantee only a modifier plus one primary (around 2KRO); three letter keys can already drop. USB full-key reports are also limited by packet length, so even a correct scan may not all reach the host. The symptom is a letter vanishing mid-word, or a game character stopping a strafe.

Studying it

Hold three keys known to form a rectangle on the target board and watch for a fourth key or a missing report. Compare with the vendor's rollover claim. For games, WASD plus space plus the keyboard equivalent of a mouse side button. A logic analyzer can see the matrix; a software key-test page is enough to show designers the phenomenon. Do not call USB-polling latency a dropped key.

Where it stops holding

Mechanical boards with diodes almost never drop ordinary typing chords; membranes and laptop islands concentrate the problem. Soft keyboards have no such matrix; their failure is contact merging, not ghosts. Remote desktop forwards already-dropped events; the far end cannot restore them. Intentional macro keys are not drops.

Applying it

• For games and frequent chords, document that 6KRO or n-key rollover is required; do not assume an office membrane is enough.
• Avoid shortcuts whose three physical keys form a matrix rectangle.
• Ship a key-test page so users can see which combinations drop.
• Verify on the shipping keyboard: hold WASD plus common action keys; the test page should show every down. A missing key means change the board or add diodes, not blame the player's speed.

Related

• Same group: C6.31.1 Holding a character key starts repeat; repeat rate and initial delay are independent parameters · C6.31.2 A repeat rate that is too fast produces extra characters when the user wanted one · C6.31.4 N-key rollover reports any simultaneous combination correctly; games and pro boards rely on it
• Adjacent: C6.20 Modifiers and key combinations · C6.02 Key travel and tactile confirmation
• Search: key ghosting · blocking · keyboard matrix