Low power dissipation touch plane interface circuit

Details Low power dissipation touch plane interface circuit Low power dissipation touch plane interface circuit

2000-09-29

2004-06-22

Hjerpe, Richard (Department: 2674)

Computer graphics processing and selective visual display system

Display peripheral interface input device

Touch panel

C178S018050

Reexamination Certificate

active

06753853

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to interface circuits for touch screens. This invention also relates to methods of processing inputs from touch screens. This invention also relates to integrated circuits that include interfaces for touch screens.
2. Description of Related Art
Touch plane operator input devices, such as touch screens and touch pads, are known. Typically, a touch plane operator input device provides a generally planar surface that is sensitive to the touch of an operator and is operative to provide one or more output signals indicative of the location of the touch on the plane. The output signals may be based either on the raw data from a touch screen sensor system, or may be based on processed data that provides X-Y coordinate information of the touch.
Touch screens are an enhanced type of computer display device that include a touch plane operator input device. Touch screens are therefore capable not only of displaying information to an operator, but also of receiving inputs from the operator. Touch screens have been put to use in a wide variety of applications. Such applications include consumer applications such as personal digital assistants (PDAs), digital audio playback systems, internet devices, and so on, as well as industrial applications such as operator interfaces in industrial control systems. In some applications, the operator touch is made by a stylus or other device held by the operator. In other applications, the operator touches the screen directly.
Touch pads are similar in operation to touch screens, except that they are not used in connection with a display device. Touch pads are often placed adjacent the space bar on laptop computers to allow operator control of a mouse pointer. Numerous other applications also exist.
For convenience, the discussion will now focus on touch screens, it being understood that the discussion is equally applicable to touch pads and other touch plane operator input devices. In many touch screen systems, a computer system is implemented using “system-on-chip” integrated circuits. In a single chip, these integrated circuits provide many of the functions that used to be spread among many integrated circuits. For example, in addition to the main microprocessor, it is not uncommon to have other circuits such as specialized serial interfaces, UARTs, memory controllers, DMA controllers, Ethernet interfaces, display interfaces, USB (universal serial bus) interfaces, and so on, as well as a touch screen interface used to acquire data from a touch screen.
A problem that has been encountered with system-on-chip integrated circuits adapted for use with touch screens is that there are many different types of touch screens. For example, some touch screens are relatively small (e.g., three inches or less) whereas other touch screens are much larger (e.g., twenty inches or more). The interface characteristics of large touch screens tend to be different because voltage feedback provisions are made to compensate for the effects of resistance and temperature drift due to the larger screen size. Additionally, even within the feedback
onfeedback categories of touch screens, variations exist. As a result, it has been difficult to provide a system-on-chip that is usable in a wide variety of touch screen applications because different touch screen applications tend to use different types of touch screens and different types of touch screens have different interface characteristics.
FIGS. 1A-1D
below show four different types of commonly employed analog resistive touch screens. In general, most analog resistive touch screens comprise front and back resistive layers (often formed of indium tin oxide) that are pressed together when an operator touch is received. The operator touch causes the two layers to establish an electrical contact at a particular location on each layer. Therefore, by applying a voltage to one layer and reading the voltage established by electrical contact on the other layer, the location of the touch can be determined based on the known characteristics of each layer.
For example,
FIG. 1A
is a schematic diagram of a 4-wire analog resistive touch screen. As shown therein, the touch screen comprises an X-axis resistive layer
12
and a Y-axis resistive layer
14
. The resistance of the layers
12
and
14
is shown schematically as four resistors. The X-axis layer
12
further includes an X+ bus bar
16
that connects to an X+ terminal
18
of the touch screen, and an X− bus bar
20
that connects to an X− terminal
22
of the touch screen. Similarly, the Y-axis resistive layer further includes a Y+ bus bar
26
that connects to a Y+ terminal
28
of the touch screen, and a Y− bus bar
30
that connects to a Y− terminal
32
of the touch screen. The touch screen is scanned in the X-direction by applying a voltage across the X+ and X− bus bars
16
and
20
, and then sensing the voltage that appears at one or both of the Y+ and Y− terminals
28
and
32
. Assuming negligible current flow through the Y+ and Y− terminals, the voltage at the Y+ and Y− terminals
28
and
32
should be approximately the same and is determined by the X-coordinate of the point of electrical contact between the X-axis and Y-axis layers
12
and
14
, that is, by the X-coordinate of the touch. By comparing the voltage to values determined during calibration, the X-coordinate of the touch can be determined. The Y-coordinate of the touch is then determined in the same manner, except that a voltage is applied across the Y+ and Y− bus bars
26
and
30
, and the resultant voltage that appears at one or both of the X+ and X− terminals
18
and
22
is sensed. Of course, with all touch screens, X and Y axis definitions are arbitrary and different definitions can be coordinated with program code to determine screen position.
FIG. 1B
is a schematic diagram of an 8-wire analog resistive touch screen. The 8-wire touch screen is the same as the 4-wire touch screen, except that four additional sX+, sX−, sY+ and sY− feedback terminals
40
-
43
are provided. Typically, both 4-wire touch screens and 8-wire touch screens use an analog-to-digital converter to sense the voltages that appear at the X+ and Y+ terminals; In the case of a 4-wire touch screen, the reference voltage inputs to the analog-to-digital converter are connected directly to the same positive and ground terminals of a power supply that also applies voltages to the touch screen. In the case of an 8-wire touch screen, the reference voltage inputs are connected to sX+ and sX− terminals
40
and
42
of the X+ and X− bus bars or to sY+ and sY− terminals
41
and
43
of the Y+ and Y− bus bars, respectively. The sX+, sX−, sY+ and sY− terminals
40
-
43
are used for voltage feedback to eliminate the effects of resistance and temperature drift in the circuit components.
FIG. 1C
is a schematic diagram of a 5-wire analog resistive touch screen. The 5-wire analog resistive touch screen includes a resistive layer
52
and a wiper layer
54
. The resistive layer includes V+, V−, Z+/−, and Z−/+ terminals
56
-
59
at the four opposing corners of the touch screen. A constant voltage is applied to the V+ and V− terminals
56
-
57
. The X and Y axes are scanned by applying a voltage at the Z+/Z− and Z−/Z+ terminals
58
-
59
, and then reversing the polarity of the voltage to scan the other direction. The resulting two voltages produced at the wiper terminal
60
are indicative of the X and Y-positions of the touch.
FIG. 1D
is a schematic diagram of a 7-wire analog resistive touch screen. The 7-wire touch screen is the same as the 5-wire touch screen, except that two additional sV+ and sV− feedback terminals
61
-
62
are provided. As with the sX+, sX−, sY+ and

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