Coded data generation or conversion – Bodily actuated code generator – Including keyboard or keypad
Reexamination Certificate
2001-05-07
2004-08-31
Wong, Albert K. (Department: 2635)
Coded data generation or conversion
Bodily actuated code generator
Including keyboard or keypad
C345S168000
Reexamination Certificate
active
06784810
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the field of monitoring the operation of input devices. More particularly, the invention relates to an apparatus and a method for providing a fast, low power consumption, detection of at least one depressed key in a resistive matrix keyboard.
BACKGROUND OF THE INVENTION
Keyboards are widespread as a standard input device for many electronic devices. In Personal Computers (PC), for instance, the keyboard is the main input device available for users to enter data and interact with the PC.
PC keyboards usually comprise keys for typing, numbers, alphabetic letters, algebraic operators, special functions and more. Because of the large number of keys, it is customary to arrange the keys in a matrix, each key being implemented by a switch that has a unique two-dimensional location of the matrix, indicated by a row and column number. When a key is pressed, the mechanical operation is transferred to a corresponding switch, which in response changes its state to a conducting state. In order to detected depressed keys, the matrix is scanned to reveal switches that are closed (conducting), corresponding to a pressed key. A typical conventional keyboard further comprises an encoder, which translates the location (row and column) of the closed switch in the matrix into a code symbol that is later used to uniquely identify the depressed key.
FIG. 1
schematically illustrates a prior art keyboard matrix depicting the matrix scan procedure that is required to detect a depressed key. The matrix is constructed from horizontal and vertical conducting lines, which form the matrix rows and columns, respectively. Two columns (KBSout
1
, KBSout
2
) are the matrix's drive lines, and two rows (KBSin
1
, KBSin
2
) represent the sense lines of the keyboard matrix. KBSout
1
, and KBSout
2
are grounded through the N-channel MOSFET transistors, Q
1
and Q
2
, respectively. Each transistor implements an Electronic Switch (ES), which is activated when a Scan Pulse (SP) is applied to its gate.
The rows and columns junctions of the keyboard matrix in
FIG. 1
are coupled by the Key Switches (KS): KS
11
, KS
12
, KS
21
, and KS
22
. The matrix is scanned column by column by applying an SP to each drive line ES, one at a time. When an SP is applied to one of the drive lines ES (e.g., ES
1
), the corresponding electric switch that is connected to that drive line is turned ON (i.e., Q
1
is switching to its conducting state), thereby enabling electric current flow through that drive line to ground. If all the KSs are open, then no current flows and the voltage level of all the sense lines equals V
CC
(normally a “1” logic). If a KS
ji
on the scanned drive line is closed (i.e., a pressed key), when ES
i
is applied with a Scan Pulse (SP) KBSout
i
is pulled to LOW. Consequently, the electric voltage on the KBSin line that corresponds to the row that contains the closed switch (KBSin
j
) drops to a lower level voltage V
d
, thereby indicating the row number (i.e., j) and the column number (i.e., i) of the closed switch.
The value of the resistors Rp and the transistors' on voltage (V
d
) are designed such that the row voltage V
j
(v
j
=V
d
, j=1, 2) is identified as a “0” logic by the input gate on KBSin
j
. This way, the depressed keys' rows and columns are located. For example, if an SP is applied to drive line KBSout
1
(i.e., to ES
1
), and KS
11
is closed, the voltage on sense line KBSin
1
drops from V
CC
to V
d
, indicating a key closure between the KBSout
1
column and the KBSin
1
row.
False detection occurs when several keys are simultaneously pressed. This is known in the art as the “phantom key” problem, that is, when closed circuits are formed by several KS being pressed simultaneously, which results in false indication of pressed keys. For example, if KS
11
, KS
12
, and KS
22
of the keyboard matrix are simultaneously pressed when the SP is applied to KBSout
1
(ES
1
), the current that flows through Rp
2
, KS
22
, KS
12
(in the reverse direction) and KS
11
to ground, causes a voltage drop (from V
CC
to V
d
) on KBSin
2
, resulting in false indication of KS
21
as closed.
Solving the “phantom key” problem is necessary to achieve a proper detection whenever several keys are simultaneously pressed. This problem is also known as “n key rollover.” The keyboard encoder is designed to resolve “n key rollover” conditions and “phantom” keys, and in addition, to perform switch contact debouncing.
A prior art implementation that resolves “phantom” key problems is depicted in
FIG. 2
, which illustrates a keyboard construction with diodes connected in series to each KS of the keyboard matrix. The diodes in this implementation eliminate the “reverse” current flow through KS
12
(with reference to the previous example) and therefore prevent the “phantom key” problem. In this way, “n key rollover” is prevented. This solution does have the side effect that the diodes forward drop adds to V
d
, which forces a higher value for the highest voltage that is still considered as a “0” logic state of the KBSin inputs. In addition, keyboard parts count is substantially increased (series diodes) resulting in an expensive assembly of the keyboard.
Another keyboard implementation that resolves the “n key rollover” problem is the resistive matrix keyboard, depicted in
FIG. 3
, in which a closed KS presents a non-zero resistance rather than a short circuit. When a key is pressed, its resistance provides the electrical connection required for indicating a key closure. The resistive matrix outputs a set of analog signals, rather than digital logic values. An array of comparators (CMP), each of which is fed by a predetermined threshold voltage, is used to distinguish between the voltage levels cause by one, or “n key” pressing.
A diversity of analog voltage levels appears at the resistive matrix rows KBSinj, in accordance to the number of keys pressed. The voltage difference between an actual (real) KS closure and a faulty closure indication (i.e., “phantom” key) is relatively small, so that the threshold voltage level (V
ref
) should be carefully determined to ensure a correct detection of the pressed keys by the set of comparators. However, deviations of the KS resistance values in the matrix result in changes of the voltage levels, which may be a source for faulty KS closure detection. The voltage level variation is affected by tolerances of the KS resistance values during production and by environmental effects like temperature, moisture, dust and corrosion of the contacts. Moreover, the threshold voltage levels usually fluctuate over time and temperature, contributing more imprecision.
In the design and manufacture of a portable PC (e.g., a “laptop”), a great effort is invested to substantially reduce the device's size, weight, and power consumption. Reduction of the power consumption is essential to achieve a long term operation of electronic mobile and wireless devices operating from a battery power supply. In order to reduce the device's size, and increase the efficiency, an embedded controller (EC) is utilized to interact with the laptop's input/output (I/O) devices.
The power consumption of the EC is influenced by the controller operation. It is desired to reduce the controller activity to a minimum by executing a fast matrix scan and by halting the controller operation when it is not needed. When halted, the system can be in a standby mode, in which the matrix is only sensed to detect a press condition on any of the keys, or on a specific subset of them, as an indication for activity. This is known as “any key pressed” detection, and it is performed by applying one SP to all the drive lines and testing the sense lines voltage levels. If a key is pressed it is detected by a logic “0” in one of the KBSin inputs, this indication is utilized as a wake event to the EC. In response, once the activity in the EC is resumed, it will use the normal key scan process to detect exactly which key is pressed.
The Mitsubishi 3886
Falik Ohad
Flachs Victor
National Semiconductor Corporation
Wong Albert K.
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