Editing Rollover, blocking and ghosting (section)

==Ghosting==

Ghosting is a potential issue where certain combinations of three or more keys can cause an additional key to be registered. Ghosting is a rare problem, and is a sign of a serious design flaw in the keyboard. Many manufacturers misuse the term "ghosting" to refer to blocking, or use the term "anti-ghosting" to refer to a gaming optimized matrix.

To simplify the circuitry, the [[key]]s on the keyboard are wired into a grid, known as the [[keyboard matrix]]. The [[keyboard controller]] applies current to each column in the matrix in turn, and checks to see which rows output current. This indicates which grid intersections within that column are closed switches, i.e. depressed keys. Ghosting occurs when current flows the wrong way around the matrix and causes a row to output current when it should not.

The following highly simplified example matrix shows three keys depressed, W, E, and D:

[[File:Matrix scan current flow without diodes.svg]]

The matrix has applied current to column B to test which keys in that column are depressed. Current is flowing through switch B1 and out of row 1, so the B1 intersection—the W key—is detected.

Current also flows the wrong way through switch C1, then down through C2 and out of row 2. Since the keyboard controller is only applying current to column B, it mistakes current in row 2 as being an indication that switch B2 in that column—the S key—is depressed. This switch is the "ghost": the computer will see the S key (switch B2) as being depressed when it is not.

This reverse current flow can be prevented in some keyboards with the use of diodes. Diodes are electronic circuit components that only pass current in one direction. The diode protecting switch C1 prevents current flowing through it, and switch B2 is no longer mistakenly detected:

[[File:Matrix scan current flow with diodes.svg]]

A [http://www.dribin.org/dave/keyboard/one_html/ longer explanation of this process] can be found at dribin.org.

Diodes can only be used on keyboards which have a rigid printed circuit board. Membrane keyboards such as [[rubber dome]] and most [[buckling spring]] keyboards have nowhere to mount diodes, and cannot support diode protection. Capacitive keyboards do not have electrical switches and as such are immune to this problem from the start, although typically they do have a PCB.

Keyboards without a PCB solve this problem using blocking: the controller will limit the number of keys pressed concurrently to a safe limit. Since ghosting occurs when three out of four keys are pressed within a square section of the matrix (which typically does not correspond to a group of four physical keys), the safe limit is considered to be two keys: any third key pressed after that is simply not registered, to protect against the third key causing ghosting to occur. Such keyboards are referred to as '''two-key rollover''', or '''2KRO'''. Keyboards with diode protection that can detect an unlimited number of simultaneous keypresses are termed '''N-key rollover''', or '''NKRO'''.