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UR5HCFJ8-FB

Low-Power Keyboard Encoder for Portable Systems

Semtech Corporation 

KEYBOARD ENCODER, (CON’T)

Embedded Numeric Keypad

The UR5HCFJ8 implements an

embedded numeric keypad. The

Numeric Keypad Function is

invoked by pressing the Num Lock

Key.

FN Key

A special FN Key has been

implemented to perform the

following functions while it is held

pressed:

• Function Key F1 becomes F11

• Function Key F2 becomes F12

• Ctrl Left Key becomes Ctrl Right

• Alt Left Key becomes Alt Right

If Num Lock is set:

• Embedded numeric keypad keys

become regular keys.

If Num Lock is not set:

• Embedded numeric keypad keys

provide the same codes as a

numeric keypad when the Num

Lock is not set (Arrow Keys, PgUp,

PgDn, etc.)

Status LED indicators

The controller provides interfacing

for three LED shift status indicators.

All three pins are active low to

indicate the status of the host

system (Num Lock, Caps Lock and

Scroll Lock). They are set by the

system (AT/PS/2 protocol).

MODE CONTROL

Operating modes are defined by the logic level of the mode pin in the Mode

Control Unit.

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 code(s) will be transmitted while that key is held pressed.

When the 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

to the number of keys that can be held pressed at the same time. However,

two or more key closures, occurring within a time interval of less than 5 mS,

will set an error flag and will not be processed. This procedure protects

against the effects of accidental key presses.

“Ghost” Keys

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

rectangle on the switch matrix are held 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

Actual key presses

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

“Ghost”

time by a fast typist (i.e., keys located

Key

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.

Figure 1: “Ghost” or “Phantom” Key

Problem

The UR5HCFJ8 has built-in mechanisms to detect the presence of “ghost”

keys, thus eliminating the necessity of external hardware.

Copyright Semtech, 1997-2001

4

DOC5-FJ8-DS-107

www.semtech.com

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