SPECIAL HANDLING

MODE CONTROL

Copyright ©1997-2001 Semtech Corporation

DOC5-418-DS-109

www.semtech.com

4

Connection of External

Device

presence of an external device. If

an external keyboard or other

device is not connected at power-on

and is connected at a later time, the

encoder proceeds with the normal

reset routine in order to properly

initialize the external device. After

communication has been

established, the encoder continues

to check for the presence of the

external device. If the device is

disconnected at a later time, the

encoder becomes aware of it. If a

subsequent connection takes place,

the controller re-initiates a reset

sequence. This unique feature

allows the user to connect or

disconnect an external device at

any time without having to reset the

system.

Shift Status LEDs

Shift Status LEDs (Num Lock, Caps

Lock and Scroll Lock) indicate the

status of the system and are

controlled by commands sent from

the system. Set/Reset Status

Indicator Commands from the

system will be executed both by the

external keyboard and the scanned

matrix.

For example, if the user presses the

Caps Lock Key on either keyboard,

the Caps Lock LED will be effected

on both keyboards. The LED status

indicators are properly set after

each new connection of an external

keyboard.

N-Key Rollover

In this mode, the code(s) corresponding to each key press are transmitted

to the host system as soon as that key is debounced, independently of the

release of other keys.

If a key is defined to be Typematic, the corresponding make code(s) are

transmitted while the key is held pressed. When a key is released, the

corresponding break code(s) are then transmitted to the host system. If the

released key happens to be the most recently pressed, then Typematic

action is terminated. There is no limitation in the number of keys that can

be held pressed at the same time. However, two or more key closures,

occurring within a time interval less than five mS, set an error flag and are

not processed. This procedure protects against effects of accidental key

presses.

“Ghost” Keys

In any scanned contact switch matrix, whenever three keys defining a

rectangle on the switch matrix are pressed at the same time, a fourth key

positioned on the fourth corner of the rectangle is sensed as being

pressed. This is known as the “ghost” or “phantom” key problem. Although

the problem cannot be totally eliminated without using external hardware,

there are methods to neutralize its negative effects for most practical

applications. Keys that are intended to be used in combinations or are

likely to be pressed at the same time by a fast typist (i.e., keys located in

adjacent positions on the keyboard) should be placed in the same row or

column of the matrix whenever possible. Shift Keys (Shift, Alt, Ctrl) should

not reside in the same row (or column) with any other keys.

mechanisms to detect the presence of

a “ghost” key, thus eliminating the

necessity of external hardware.

Actual key presses

“Ghost”

Key

Figure 1: “Ghost” or “Phantom” Key

Problem