Capacitively coupled keypad structure

Details Capacitively coupled keypad structure Capacitively coupled keypad structure

2000-12-21

2003-10-14

Horabik, Michael (Department: 2635)

Coded data generation or conversion

Bodily actuated code generator

Including keyboard or keypad

C341S020000, C341S022000, C345S168000, C345S172000, C379S369000, C379S433070, C379S370000, C361S689000

Reexamination Certificate

active

06633241

ABSTRACT:

TECHNOLOGICAL FIELD
The invention concerns generally the electromechanical structures of keyboards. Especially the invention concerns the integration and coupling of keyboard structures into the other electrical and mechanical parts of an electronic device.
BACKGROUND OF THE INVENTION
A keypad consisting of individually pressable numeric or alphanumeric keys as well as function and control keys is an important part of many modern electronic devices. Especially keyboards of this kind appear in portable telecommunication devices such as mobile telephones and multi-function communicators, portable digital assistants and palmtop computers.
FIG. 1
is an exploded cross-sectional view that shows a known structural arrangement which is built on a printed circuit board or PCB
101
with a number of contact pads on its upper surface. An insulating film
102
covers the PCB except of those contact pads onto which components are to be placed. The keyboard part comprises a dome sheet
103
(or alternatively a selection of individual key domes) made of some resilient material that is electrically conductive at least at the concave surface of each dome. Additionally the keyboard part comprises a rubbery keymat
104
where the bulging protrusions coincide with the conductive domes of the dome sheet
103
and constitute the visible parts of the keys. A microphone
105
is coupled to its contact pad(s), and the keyboard-microphone entity is covered by an ESD shield
106
which is made of a thin sheet of stainless steel or other conductive material.
The electric implementations of keypads come in various forms.
FIG. 2
illustrates a known rectangular keypad that consists of N×M keys; here N=4 and M=5. The arrangement comprises N conductive input rows
201
to
204
and M conductive output columns
205
to
209
where normally the rows and columns do not have a galvanic contact with each other. At each junction of a row and a column there is a key comprising a conductive dome shown as a dotted circle. Pressing for example the key shown as
210
will cause the conductive dome to produce a galvanic contact between row
204
and column
209
. An input voltage distributor
211
energises each of the input rows in cyclical turns at a rate that is expected to exceed the rate of pressing the keys by the user. An output detector
212
detects the energisation caused in one of the output lines when the energisation of the corresponding input line coincides with a conductive dome coupling the input and output lines at one intersection. The logical encoding block
213
converts the observation of a keystroke to a bit sequence that unequivocally indicates, which key was pressed.
The drawbacks of the arrangement of
FIG. 2
comprise the relatively high power dissipation of the N parallel voltage buffers included in block
211
that are required to maintain the continuous sequential energisation of the input rows.
From the European patent publication number EP 695 041, which is incorporated herein by reference, there is known a keypad arrangement according to
FIG. 3. A
supply voltage Vcc is provided to a positive voltage rail
301
through a resistor
302
. From the positive voltage rail there is a connection to each column of keys through a resistor
303
-
306
so that each of the resistors has a different value. Each row of keys is coupled to a ground rail
307
so that from each row there is a connection to a common connection point through a differently valued resistor
308
-
312
, and from the common connection point there is a connection to the ground rail through a resistor
313
. The positive voltage rail
301
is coupled to a first output port
314
. The circuit comprises also a PNP transistor
315
the emitter of which is coupled to the supply voltage Vcc, the base of which is coupled to the positive voltage rail
301
through a resistor
316
and the collector of which is coupled to the ground rail
307
through a resistor
317
. The collector of the PNP transistor
314
is also coupled to a second output port
318
.
The idea of the keypad according to
FIG. 3
is that when a certain key is pressed, a resistive current path is produced between the positive voltage rail and the ground rail. The resistance value of the current path is unambiguously associated to the depressed key, because all the column resistors
303
-
306
and row resistors
308
-
312
have different values. The variable resistance of the current path acts as a part of a voltage divider together with the resistor
302
, which in turn causes the positive voltage rail to assume a voltage value that is also unambiguously associated to the key which was pressed. An A/D converter (not shown) connected to the first output port converts the voltage value of the positive voltage rail into a digital word representing the depressed key. The circuit built around the PNP transistor will generate an interrupt signal that is readable from the second output port and indicates when a key is pressed.
From the European patent application number 99660183.7, which is incorporated herein by reference, there is known a keypad arrangement the functional principle of which is illustrated in FIG.
4
. The arrangement comprises a first resistive strip
401
and a second resistive strip
402
in a parallel, non-touching set-up. At point
403
there is a temporary conductive connection between the resistive strips, caused by for example a conductive dome being pressed simultaneously against the surfaces of both resistive strips. The first end (the left-hand end in
FIG. 4
) of the first resistive strip
401
is coupled to a supply voltage Vcc through a resistor R
1
and the second end of the resistive strip
401
is coupled to the ground potential. Correspondingly the first end of the second resistive strip
402
is coupled to an output port for an output voltage Vout to be measurable and the second end of the second resistive strip
402
is not coupled to anywhere.
When one knows the specific resistances of the resistive strips
401
and
402
and the distances X and Y, it is possible to calculate the resistance values of the pieces of resistive strips on each sides of the connection point. By assuming that the output port where the output voltage Vout is measured sinks only a negligible current, it is possible to further calculate the output voltage Vout which turns out to depend only on the position of the conductive connection
403
in relation to the ends of the first resistive strip, as well as on the specific resistance of the strip. Detection of individual key presses boils down to the task of measuring an analogue voltage value. The application EP 99660183.7 suggests various meandering and other shapes to the resistive strips on the surface of a printed circuit board or the like.
As long as the electrical implementations of
FIGS. 2
,
3
and
4
are based on the mechanical structure shown in
FIG. 1
, they all share the disadvantage of requiring a relatively large area from the printed circuit board. Additionally they tend to impose limitations to the layout design of the printed circuit board even though the principle shown in
FIG. 4
allows for considerably more flexible implementations in this respect than the other solutions.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a keypad structure which is advantageous in terms of manufacturing and places only few limitations to the layout design of both the printed circuit board and the appearance of the keypad itself. It is also an object of the invention to provide a keypad structure that allows reliable and accurate detection of key presses and is tolerable to statistical variation in component characteristics and changes in environmental conditions.
The objects of the invention are achieved by building the keypad as a part of a cover part and by using a suitable coupling to realize the electrical connections between the keypad and other electronic parts of the device in question.
The keypad arrangement according to the invention comprises
a keypad co

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