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KEYBOARD ENCODER, (CON’T) MODE CONTROL Copyright Semtech, 1997-2001 DOC5-FJL-DS-106 www.semtech.com 4 Embedded Numeric Keypad The GreenCoderTM 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 • Control Left Key becomes Ctrl Right • 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 an interface 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) and are set by the system. After approximately a one- minute period of keyboard inactivity, LEDs are dimmed to conserve power. They are set to full brightness again upon a new keystroke. 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) will be 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 5 mS, will set an error flag and will not be 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 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 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 GreenCoderTM 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 |
