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| 型號： | SH703600FG |
| 元件分類： | 微控制器/微處理器 |
| 英文描述： | SPECIALTY MICROPROCESSOR CIRCUIT, PQFP32 |
| 封裝： | 7 X 7 MM, 0.80 MM PITCH, PLASTIC, LQFP-32 |
| 文件頁數(shù)： | 6/11頁 |
| 文件大小： | 109K |
| 代理商： | SH703600FG |
Copyright 1999-2004 Semtech Corporation
SH703600 data sheet v1.02 (2004-10-28)
www.semtech.com
4
“GHOST” KEYS
KEYBOARD SCANNER
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.
Figure 1: “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 should be placed in
the same row or column of the
matrix, whenever possible. Shift
keys (Shift, Alt, Ctrl, Window)
should not reside in the same row
(or column) as any other keys. The
SH703600 has built-in mechanisms
to detect the presence of most
“ghost” keys.
Actual key presses
“Ghost”
Key
The encoder scans a keyboard organized as an 8 row by 16 column matrix
for a maximum of 128 keys. Smaller size matrixes can also be
accommodated simply by leaving unused pins open. The SH703600
provides internal pull-ups for the row input pins. When active, the encoder
selects one of the column lines (C0-C15) every 512 S and then reads the
row data lines (R0-R7). A key closure is detected as a zero in the
corresponding position of the matrix.
A complete scan cycle for the entire keyboard takes approximately 9.2 ms.
Each key found pressed is debounced for a period of 20 ms. Once the key
is verified, the corresponding key code(s) are loaded into the transmit buffer
of the serial communication channel.
N-key rollover means the code(s) corresponding to each key press are
transmitted to the host system as soon as that key is debounced, indepen-
dent of the release of other keys.
When a key is released, the corresponding break code is transmitted to the
host system. Several keys can be held pressed at the same time.
However, if two or more key closures occur within a time interval of less
than 5 ms, an error flag is set, and those closures are not processed. This
feature protects against the effects of accidental key presses.
N-KEY ROLLOVER
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