Designing Touch Writing Pads

Every electronic design, ranging from portable gadgets like PDA, GPS Gizmos, digital cameras, MP3 and DVD players, to music systems, point of sale terminals, and LED lighting controls, demands soft touch, modern looking, sleekly figured interactive panels for user control. Low power touch sensing is making its winning position in the user interface design window with its tactile and stylish electronic interaction experience .

Design interest and technologies
Expanding beyond the consumer market, touch sensing is also beginning to take hold in medical, industrial, and automotive applications for aesthetic, maintenance, cost, and sanitary reasons.

Touch phenomena is coming up with several technologies like capacitive, resistive, inductive, surface acoustic wave, and infrared. Each design technology has distinct advantages and disadvantages. Capacitive technology, based on electrode design in PCB, is very popular for its applications like touch keys, sliders, and wheel functionalities where the easy finger touch adds to user’s positive experience. Surface acoustic wave touch technology, based on sound waves, finds its way in designs with light transmission requirements with immense clarity like those in amusement parks and high traffic indoor environments. Infrared touch technology, based on light interruption, is used in very large frame screens with low resolution. Inductive touch screen technology is focused for use in environments where front panels are made of materials like plastic, aluminum, or stainless steel, or where surfaces can encounter liquids.

Resistive touch screen technology is cost-competitive as compared to other touch technologies, and it is very easy to integrate in an embedded systems design. This mainly caters to the segment of touch panels upto 19 inches. Support for both the finger touch detection and the pointed stylus touch detection widens the scope for resistive touch control in consumer applications (Figure 1).


Figure 1: Finger and stylus detection makes resistive touch more user friendly.
(Source: STMicroelectronics)

In this article, we will focus on resistive touch screen technology features, implementation concerns, and potential applications.

Understanding design requirements for resistive touch sensor controller selection
Since resistive touch screens are now becoming easily available and their prices are also reducing with time, it is no wonder that the scope of resistive touch user interface design is expanding. To make the best touch technology choice for an application, the application requirements should be analyzed in depth. Resistive technology offers an easy and simple PCB design since no electrodes design or coil etching is required on the PCB, which is required in capacitive and inductive respectively. Since the touch screen overlays on the display directly, it also saves the PCB space required for mechanical switches or capacitive touch key electrodes. A resistive touch screen is not recommended for use in harsh environments where explosions can occur or where an excess of dust particles are present such as mining and construction sites. Even minor damage in a resistive touch screen can hamper touch detection accuracy and linearity.

For widely used consumer products like smart phones, home automation systems, and PDAs where the displays are small and cost is a major concern, resistive technology wins the technology selection comparison analysis.

How resistive touch screen works
- A resistive touch screen is a clear glass sheet with a touch responsive surface.
- A resistive touch screen panel is composed of two resistive (ITO – Indium Titanium Oxide) layers separated by a thin spacer layer.


Figure 2: Resistive touch screen: X coordinate measurement : Black dots indicate voltage gradient points and pink dots indicate touch contact points.

- The layers of resistive touch screen form a resistive network that acts as a voltage divider circuit for touch position detection.
- Touching the screen causes a voltage variation over the voltage dividers formed by the resistive network. This voltage change is used to determine the location of the touch to the screen.
- A touch screen controller (TSC) is used to convert the analog voltage obtained to digital touch co-ordinates. With inbuilt ADC channels, it acts as a voltmeter for the measurement of the analog voltage.
- Upon touch, the touch controller acting as voltmeter first applies voltage gradient VDD to the X+ and GND to X-. Then it probes the analog voltage on Y- resistance and converts the analog voltage to a digital value using ADC for calculating the X co-ordinate as shown in (Figure 2). In this case, Y- becomes the sense wire. Similarly, applying voltage gradients to Y+ and Y-, Y co-ordinate measurement is done.
- Some touch screen controllers also support touch pressure measurement, .i.e. Z axis measurement. For Z co-ordinates, voltage gradients are applied to Y+ and X- .

Resistive touch sensing ICs are available mainly in two forms:
- Software touch sensing solutions
- Dedicated touch screen controller ICs

In software touch sensing solutions, the MCU is mainly responsible for all touch detection and co-ordinate calculation tasks. An MCU based software algorithm performs the touch detection and co-ordinates manipulation using internal MCU ADCs for touch location voltage measurement.

In dedicated touch screen controllers, the touch screen controller provides an interrupt to the system host (MCU) for touch events and the digital data for touch co-ordinates. Then the host (MCU) reads the digital data and performs operations as expected by the user.

A design approach based on MCU parameter calculation requires a very fast MCU service to manage frequent touches. Hence, it is not a very reliable design for fast touch detection. Accuracy is also lower in such designs since data averaging and touch detect delay features are not there. Dedicated touch screen controller chips with improved data sampling, measurement averaging, touch detect delay configuration and digital touch co-ordinate calculation capabilities are true touch screen controllers. They are easy to integrate and offer a higher level of performance.

Classifications of resistive touch screens
Resistive touch screens are further classified based upon the count of sensing wires available with a touch screen. They are mainly categorized in three types: 4-wire, 5-wire, and 8-wire. A 4-wire touch screen has bus bar electrodes on two different layers (X+, X- on one layer and Y+, Y- on other layer). A 5 wire touch screen has circular electrodes (X+, X- Y+, and Y-) on the lower layer only. The upper layer is used for voltage measurement during touch, and voltage gradients are only applied to the lower layer.

An 8-wire resistive touch screen's working principle is similar to that of a 4-wire touch screen. It adds an extra reference voltage line for each of the 4 wires making 8 wires in total. The four added wires are used to offer a reference voltage to each of 4 wires. It works on a ratio-metric ADC measurement principle.

4-wire touch screens are very common in low end consumer products due to their low cost and the simplicity of their touch sensing algorithm. 5-wire and 8-wire touch screens mainly focus on expensive solutions like high- end medical instruments and crucial industrial control units.

Going beyond the menu navigation
Just mapping the touch co-ordinates to display co-ordinates opens the way to command gadgets with a simple soft touch. Menu navigation, photos and map browsing, music control, and LED dimming, as examples, can be easily done using a touch screen mounted on the application’s display panel. In such applications, one can browse using a finger or stylus. Along with generic touch functionalities, support for stylus detection adds value to product delivery of embedded design houses through new application ideas like paint brush capability in PDA and handwritten SMS.

With measurement of pressure in Z axis and flow of writing measurement in X and Y axis, handwriting recognition can be done. With embedded resistive touch pads in laptops, users can browse the PC using their fingers and sign the scanned documents using a pointed stylus pen. With touch screens mounted on e-reader panels, users can easily highlight key points on their e-books using a stylus and flip pages as is done in hard paper documents.

System arhitecture and design
The major blocks in a touch sensing solution design are touch screen panel, touch screen controller(TSC), display panel and main host processor as shown in Figure 3. The host processor can be a low end microcontroller. The host manages the touch screen controller initialization and digital co-ordinate data reading using a one or two wire (I2C/SPI) interface protocol. The host is also responsible for mapping the user’s touch to the required operation like volume up/down, image change, or writing display. Most consumer gadgets make use of display panels. The same display can be used to show icons for human/machine interaction.


Figure 3: Resistive touch sensing solution block diagram.

To design a system with resistive a touch sensing user interface, an application’s complexity depends upon its touch resolution requirement. Touch resolution further depends upon ADC resolution of the touch screen controller. Another important factor is the power consumption of the touch screen controller. It’s recommended to use a touch screen controller with an interrupt functionality and low power idle state. When there is no user interaction with the system, the system should enter a low power state, and when there is touch, it should wake up and perform the touch voltage decoding. This capability becomes a requirement in portable applications where each coulomb of charge in the battery is precious.

Using a touch screen controller with an internal buffer is very helpful for frequent touch detection. As in the case of writing, touch is continuous in flow. Hence, if a touch screen controller is comprised of a FIFO buffer, then data processing can be done after the FIFO is full. This reduces the processing overhead on the host. For large screens (>6 inches), noise picked up by the conductive planes of the touch screen could affect the touch screen’s accuracy. Capacitors could be added to the touch screen (at X+/X-, Y+/Y-) to reduce high-frequency noise.

An implementation example
Let’s analyze a low cost writing pad solution implementation (Figure 4) to have a clear understanding of a resistive touch sensing solution. In this example, writing pad implementation is done using STMicroelectronics’ advanced resistive touch screen controller, STMPE811, and host processor, 32 bit MCU STM32 high density.

This solution offers a real-time writing experience on a TFT-LCD panel to the users. The X and Y co-ordinates of the stylus pen on a 4-wire resistive touch screen are mapped to a line draw on the TFT-LCD display panel. In the present writing pad design, a 2.4” touch screen is mounted on a 2.4” (QVGA resolution) TFT-LCD panel. In most mobile phones and PDAs, lower resolution screens are used. It is important to note the resolution of the touch screen and display panel to manage accurate mapping of the touch detect to the display location. Another important point is the variation of touch co-ordinates along the axis(X/Y) of the touch screen. This depends upon the make of the touch screen. In some touch screens, co-ordinate values obtained from the TSC decrease from top to bottom along the axis of the touch screen or vice-versa.


Figure 4: Writing pad solution.

Here, the TSC is interfaced with a 32bit MCU using I2C protocol. The TFT-LCD panel is interfaced with the MCU using the flexible memory interface (FSMC) of the MCU. The MCU configures the TSC for various parameter settings like ADC sampling speed and averaging values. In addition to I2C protocol interfacing, the TSC offers an output interrupt pin for touch sense detection. The Interrupt pin of the TSC is tied to the MCU’s external interrupt port pin. The TSC used in this solution is comprised of a 12 bit ADC and a FIFO with 128 sets of touch data. When touch is detected by the TSC, the MCU gets an interrupt on its external interrupt port pin and reads the data from the TSC’s FIFO using I2C protocol. A 12 bit value is available for each X and Y co-ordinate data respectively. For software mapping of touch co-ordinate to pixel display co-ordinate, calculation is done based upon the resolution of the display panel and the resolution of the touch screen. The MCU manipulates the TFT-LCD pixel display co-ordinates and displays the corresponding TFT-LCD pixels. A 12 bit ADC resolution is quite good. Hence, very tight touch points are obtained. This enables consecutive pixels to glow, offering a real-time line draw feel to the user (Figure 5).

Due to the in-built FIFO buffer in the TSC, MCU processing overhead is easily managed. Further, a color table can be displayed on one side of the display panel. The text color can be chosen by just clicking on a color, and the next line draw will be of the chosen color. A touch icon is also provided to clear the screen when it is full. In this way, a simple paint–brush application is easily implemented. This application could be the basis for a drawing kit for a child. Using this solution, a child’s creative writing and drawing can be easily shared over the internet with doting family members.

It’s not so far-fetched that every gadget used at home, office or wherever could have touch sensors for easy human to machine interaction. Enjoy the simplicity of touch sensing.


Figure 5: Flow of writing pad implementation.


STMicroelectronics