UR5HC418 Datasheet, PDF (4/14 Pages) Semtech Corporation – AT / PS/2 - Compatible Keyboard Encoder

SPECIAL HANDLING
Connection of External
Device
The DeskCoder^TM detects the
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.
MODE CONTROL
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.
The DeskCoder^TM has built-in
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
Copyright ©1997-2001 Semtech Corporation
4
DOC5-418-DS-109
www.semtech.com