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Powering resistive touch screens efficiently

(The first half of this article appeared in print in Planet Analog Magazine, May 28, 2007)

Increasingly, engineers are designing touch screens into their portable devices, Figure 1 . Although battery technologies are becoming more efficient, it is critical to keep the power dissipation of the system as low as possible. This is critical because the sophistication of the electronics around the touch screen is growing dramatically. In addition, the end user is expecting more time between each recharge of the battery.

Figure 1. Using resistive touch screens is common in many battery powered, consumer applications.

If lower-power consumption is your goal, you will be successful if you pay special attention to your analog power-down strategies, finesse the analog-to-digital converter (ADC)/processor digital interface, and optimize the touch screen control algorithms.

This article starts by looking at the construction of a resistive touch-screen panel, while discussing the inner workings of a typical 4-wire resistive screen. From there, we outline a block diagram of a touch screen system which includes the panel, a touch-screen controller and a microprocessor. Next, we'll look at hardware low-power strategies. After the first level of the power-consumption evaluation, we'll delve into more hardware and appropriate-software strategies. Finally, we'll evaluate the digital interface from a low-power perspective.

What is a touch screen and how does it work?

Designers of consumer products can choose their panel from various touch-screen technologies. Most of the panel technologies available use resistive, capacitive, surface acoustic wave (SAW), or infrared (IR) techniques. The most popular touch screen on the market is resistive because it is inherently stable and affordable.

There are 4-wire, 5-wire, 7-wire, or 8-wire resistive touch screens. The most common resistive touch screens have four wires. The layers of a 4-wire resistive touch screen panel, from top to bottom, are a rectangular, flexible top layer; a transparent, conductive coated layer (the conductive coating is usually made of indium tin oxide or ITO); air-gap and isolation spacers; another transparent ITO layer; and finally, a stable layer. In Figure 2a , yellow outlines the top ITO layer, green outlines the second ITO layer, and blue outlines the bottom stable layer of a 4-wire touch screen panel.


Figure 2. With the 4-wire touch screen panel, use the two active areas of the resistive touch screens to sense the X and Y pressure points (a). The equivalent circuit is simply a voltage-divider (b).
(Click to Enlarge Image)

The flexible top layer of the panel (not shown) is an overlay that provides a degree of protection to the ITO layers. This layer will flex enough that it depresses and so allows the two conductive layers to touch. Unless pressure is applied, the nearly invisible layer of spacers keeps the two ITO conductive layers apart.

When you touch the flex top layer with a stylus or finger, you find the X-Y coordinates of the touch screen panel. The pressure of the stylus causes the two ITO layers to connect. After the panel is touched, you apply power to one of the two ITO layers through the silver-ink conductive bars at opposing ends of that layer. When you power one ITO layer, such as the top yellow outlined area, you use the other ITO layer (green outlined area) to probe the location of the stylus. Then you use a high-impedance ADC to convert the voltage created by the stylus touch from the unpowered layer into a digital value.

For example, if you power the X-layer from X+ to X- (yellow) with 2.5 volts and touch a stylus approximately half-way between the two X-axis conductive bars, the Y+ and Y- terminals (green) will be equal to 1.25 V. This voltage is proportional to the applied voltage across X+ and X-, as a result of the resistive voltage divider of the panel (Figure 2b ). This technique probes the X position of the stylus. The Y position is sensed when you apply power to opposing conductive bars on the Y layer. You then use the X+ or X- terminals to sense the Y position of the stylus. You can also sense the panel stylus or finger pressure. The assigned coordinates for pressure measurements are “Z1 ” and “Z2 .”

The Resistive Touch Screen System

A touch screen system has a touch panel, a touch-screen controller, and a host processor (Figure 3 ). The touch screen or touch panel is the “resistive sensor” of the system.

Figure 3. A resistive touch screen system consists of touch screen panel, touch-screen controller, and microprocessor.

(Click to Enlarge Image)

The topology of the touch-screen controller includes a driver for the panel, a multiplexer, and an ADC. The driver in the touch-screen controller independently powers both coordinates of the touch panel to ON or OFF. The amount of current conduction through the touch panel is approximately equal to the power-supply voltage divided by the touch-panel resistance. The ADC inside the touch-screen controller measures the touch position and pressure, by converting the analog voltage from the touch screen into digital code. Typically, the ADC topology is a successive approximation register (SAR) with resolutions of 8-, 10-, or 12-bits.

There are two interfaces in the touch screen system (Figure 3): an analog interface between the panel and the touch-screen controller, and a digital interface between the touch-screen controller and host.

The touch-screen controller uses the 4-wire analog interface between the touch panel and the touch-screen controller to power the panel and execute coordinate measurements. During a given X- or Y-coordinate measurement, the touch-screen controller provides power through two wires (X+ and X-) of the analog interface to one ITO layer of the panel, and senses the coordinate location of the stylus using the second ITO layer with the other two wires (Y+ and Y-).

The digital communication between the processor and touch-screen controller includes an interrupt signal and a serial digital bus (SPI or I2 C). The processor can ignore the touch-screen controller and focus on performing other tasks, if there is no panel touch event. As soon as the panel is touched, the interrupt (Figure 3) from the touch-screen controller informs the processor. The processor then reads the touch-screen data from the touch-screen controller through the serial bus.

Touch Screen System Analog Interface

The primary factors that influence the touch-panel and analog-interface power consumption are the system power supply (VDD ), the panel resistance, and the panel ON:OFF ratio over time. The driver and ADC topology of the touch-screen controller primarily determines the limits of the system analog-interface power supply. The panel ON:OFF ratio over time is primarily determined by the settling time of the touch signal and system noise.

The range of power supply voltages for a touch screen and the CMOS 12-bit SAR ADC is from 1.2V to 5.5V. The power to the driver of the touch-screen controller has approximately the same voltage range. Depending on the particular resistive touch-screen panel, the resistance of each ITO layer can range from 100 O to several kO (Figure 2b), which also shows the equivalent circuit of a 4-wire resistive touch-screen panel. While the panel is excited, the power dissipation across the panel can range between 302.5 mW (power = 5.5V, panel resistance = 100 O) to 720 µW (power = 1.2 V, panel resistance = 2 kO). As these calculations show, power-reduction strategies include reducing the power to the panel and touch-screen controller, selecting a higher resistance panel, or both.

Because of the high value of power dissipation during panel ON time, a primary objective for this application is to keep the touch panel OFF as long as possible. As a consequence, the driver that powers the resistive panel ITO layers starts to cycle. Settling-time errors occur due to panel power-up rise time or environmental noise, such as mechanical vibrations of the touch panel, display lighting interference, system transients, electrostatic discharge (ESD), and/or electromagnetic pulses (EMP).

If this type of noisy environment exists, add noise-reduction components in the signal path between the touch panel and the touch-screen controller. The capacitor network in Figure 3 shows an example of a noise-reduction circuit. Note that any added capacitance in the input line will increase the input settling time for touch-screen controller (Figure 4 ). An increase in panel settling time decreases the sample rate of the touch-screen controller and increases the power-ON duration of the panel drivers.


Figure 4. This graph shows the rise time of the Y” line voltage from a touch panel with different line capacitances (CEX per Figure 3). Trace #4 has no CEX inserted. CE X is equal to 0.1 µF for trace #3. CEX is equal to 1 µF for trace #2.
(Click to Enlarge Image)

Figure 4 shows examples of the relationship of the touch panel's resistance and the input circuit's filter capacitor (CEX , Figure 3) over time. The time constant of rise time of the signal from the panel's touch-screen controller is equal to t or R*CEX , where R is the resistance across the touch panel from the stylus point-to-ground. The rise time of the voltage on the Y” line is equal to:

VY- (t) = VY-(FINAL) (1 – e-t/t )
where VTOUCH(FINAL ) is the final voltage at the stylus position

When the goal is to reduce touch-panel power consumption during acquisition, several factors influence your design decisions. Achieving a lower system power is accomplished over time by increasing the ratio of the panel OFF time to the panel ON time and selecting lower-resistance panels, if you use a capacitive network to reduce noise. It is true that lower resistance panels conduct more current during excitation. However, lower-resistance panels require less time to settle.

In typical screen applications, users need accurate X-, Y-, and Z-coordinate data. Accurate coordinate data requires 100 to 200 data sets per second. Touch-screen controllers, however, usually run with a much higher sampling rate. For example, if a touch-screen controller's sampling rate is 100 ksps, the touch-screen controller can sample 50,000 sets (maximum) of X/Y coordinates per second.

Touch System Digital Interface

The interface from the touch-screen controller to the host consists of a serial digital bus (typically SPI or I2 C) and an interrupt signal line. The host-processor peripheral configuration usually determines the serial interface protocol. If your host processor has an I2 C port and your system has multiple I2 C devices, you might also select the I2 C protocol to communicate with the touch-screen controller. I2 C uses fewer bus wires and has more flexibility when sharing a host I2 C port with multiple I2C devices.

An alternate serial interface is SPI. The SPI protocol data reading/writing speeds usually are faster than the I2 C interfaces. Your system may have an unused SPI port and a bandwidth-limited I2 C port. In this circumstance, the better choice of interface protocol is SPI.

Figure 5 shows the power consumption for various I2 C interface speeds:


Figure 5. This diagram shows three touch-screen controllers supplying current under different power supply voltage and I2 C bus speeds.
(Click to Enlarge Image)

From the graph in Figure 5 one might surmise that slower digital interface speeds mean lower overall power dissipation. However, a longer interface time adds additional load time to the bus line. Data transmitted with a 400 kHz bus speed transmits in just 25 percent of the time the same data transmitted using a 100 kHz bus speed. Data transmitted with a 3.4 MHz bus speed transmits in just 2.94 percent of the time that the same data takes to transmit with the 100 kHz bus speed. Basically, higher bus clocks in the touch screen system dissipate less average power.

A simple interface between the touch-screen controller and the host immediately sends the coordinate data to the host. With this type of interface, the touch-screen controller detects the event on the panel and sends an interrupt to the host. Upon receiving the interrupt, the host processor sends a power-up command to the touch-screen controller. The touch-screen controller samples the touch position, converts the signal to digital, and immediately sends the data to the host. The host controls this scheme. In this environment, the host processor may need multiple samples in order to digitally reduce noise in the panel data.

The touch-screen controller also can have primitive digital filtering capability, reducing the volume of conversion packets sent across the touch-screen controller/host interface. With this strategy, the touch-screen controller senses the pressure to the panel, acquires multiple samples for each coordinate, preprocesses the coordinate data, then sends one final solution set of coordinates to the host processor. This solution reduces the tasks that the host would otherwise perform.

Prior to the data transmission, there is minimal communication between the touch-screen controller and the host. Although the rise time of the resistive/touch screen network causes delays in analog system stability, other noise sources can have more significance on conversion accuracy. To reduce these noise sources, you may need to average multiple samples.

Figure 6 diagrams (patent pending) use three simple, but effective, filtering algorithms for a touch panel data set:


Figure 6. You can derive a median (b) or averaged (a) result from a set of touch screen data values. If you combine these two calculations, you can throw out the high and low values of the data set(c) and average a middle window.
(Click to Enlarge Image)

In all cases, initially you would sort the data from the panel in descending order. A second possible strategy is to calculate the median from the data set (Figure 6b ) and throw away extraneous data. You can apply a little more processing to the data set and average the data set, sending the results to the processor (Figure 6a ). Another alternative is to throw the low and high values from the data set out and then average the remaining values (Figure 6c ). Other debounce algorithms include voting and a processor implemented FIR filter.

The Advantage of Pre-filtering Touch Screen Data

If you use filtering to reduce noise in the ADC data, you can implement this activity in the host software. An alternative solution uses the touch-screen controller for data filtering activity. If you move the filtering activity from the host to the touch-screen controller, you can significantly reduce traffic on the digital bus lines. This change lowers the digital interface power.

For example an I2 C interface write cycle requires 18 I2 C clock cycles and one read requires 27 I2 C clock cycles. In Example A (see Table 1 ), if the touch-screen controller collects seven 12-bit samples and sends those samples to the processor, the touch-screen controller communicates with seven reads and writes.


Table 1. Comparison of the total number of I2C clocks and duration of transmission for two example systems.
(Click to Enlarge Image)

The total number of clock cycles for this interface is equal to 7 * (18 +27) = 315 I2 C clock cycles.

In Example B , the touch-screen controller collects and preprocesses the same seven 12-bit samples. Again, a touch-screen controller read requires 18 I2 C clock cycles, and a write requires 27 I2 C clock cycles, but now the total number of I2 C cycles is 1 * (18 + 27) = 45 I2 C clock cycles (see Table 1). This reduces the number of I2 C clock cycles by seven times.

It's an advantage when the touch-screen controller senses the touch of the stylus, collects the multiple coordinate data sets, preprocesses the data to reduce system noise, and then transmits the final results to the microprocessor. The duration of one I2 C clock cycle operating at a 400 kHz clock rate is equal to 2.5 µs. Under these conditions, the touch screen system in Example A requires 787.5 µs to transmit panel data. Given the conditions described in Example B, the total interface time reduces to 112.5 µs.

Similar traffic reduction happens with a digital SPI interface. With the SPI interface, the transmission of one read cycle requires 24 clock cycles. The transmission of seven unfiltered read cycles require 168 clock cycles. A touch-screen controller with built-in filtering features, transfers only one set of coordinate data through the SPI bus. This is an 86 percent reduction in the bus traffic.

A human interface can potentially create a degree of uncertainty. This can occur as a result of stylus bouncing (too much morning coffee!) or simply an unintentional brushing of the touch screen. If the touch screen system responds (with an interrupt) immediately, without verification, the touch-screen controller powers the panel under the condition of a false alarm. The system can reduce the probability of reacting to a false alarm by acquiring multiple interrupts from the panel before awarding legitimacy. For instance, the touch-screen controller can implement preset thresholds to check the data's validity. This avoids erroneous data transmissions to the host, and further reduces digital interface bus traffic.

Total Power Consumption of a Simple System

The three regions in the touch-screen controller system that consume power are the analog interface, the touch-screen controller (quiescent current and ADC), and the digital interface. In the system described in Table 2 , the analog and digital power supply are equal to 1.8V and the quiescent current of the touch-screen controller is 360 µA.


Table 2. The analog interface dominates the power consumption of the touch screen system unless the host performs the data filtering function.
(Click to Enlarge Image)

In the system in Table 2, the digital interface, power dissipation is nearly equal to the power dissipation of the analog interface and the touch-screen controller, as long as the host performs the digital filter function. If the touch-screen controller pre-filters the data before sending the final coordinate points to the host, the power dissipation of the digital interface decreases dramatically.

A Touch Screen System circuit

Figure 7 shows a working circuit diagram of a touch screen system. The touch-screen controller, TSC2005, has an array of features that enhance both the analog and digital interface.


Figure 7. Touch screen system using Texas Instrument's TSC2005.
(Click to Enlarge Image)

Analog interface enhancements include numerous delay functions, ratiometric measurement capability, and ESD/EOS protection. With one of the delay functions inside the TSC2005, you can delay the start of an acquisition and bypass the preliminary touch panel “bouncing” and/or the panel RC rise time. These drivers employ ratiometric techniques (U.S. patent no. 6,246,394) to increase the dynamic range of the ADC. Another analog feature that the TSC2005 provides is a high degree of touch panel ESD and EOS protection.

Digital interface enhancements of the TSC2005 include a filtering or averaging module. The built-in filtering algorithms of the TSC2005 minimize the digital interface traffic, reducing the host operating time and memory.

Conclusion

Designers of consumer products can select from various touch screen technologies when choosing their panel for their application. The most popular touch panel on the market is the four-wire resistive. The resistive touch screen system comprises a resistive touch panel, a touch-screen controller, and a host processor. By using the touch screen, you can determine the X-Y and Z coordinates of a stylus or finger.

The analog interface between the touch panel and the touch-screen controller has the highest impact on the power consumption in the touch screen system. The primary factors that influence this analog interface are the system power supply (VDD ), the panel resistance, and the panel ON:OFF time ratio. The general guidelines for reducing power consumption of the panel and analog interface, if there are no settling time issues, is to reduce the system power, use a higher resistance touch panel, and keep the ON:OFF time ratio of the application low. If you add a noise-reduction, capacitor network to the circuit, lower resistance touch panels will dissipate lower overall power versus time.

The primary power factor that influences the touch-screen controller and host processor digital interface is serial bus power dissipation caused by high digital traffic. Averaging the digital conversion results in the touch-screen controller reducing the impact of the host processor, and digital interface power dissipation. If the touch-screen controller filters the coordinate data before transmission to the host, the most significant power consumption is not the digital interface between the touch-screen controller and the host. The touch screen consumes a considerable amount of power through the analog interface when the panel is ON.

References:
1. “Using resistive touch screens for human/machine Interface,” by Rick Downs, Analog Applications Journal, 3Q, 2005, Texas Instruments: http://focus.ti.com/lit/an/slyt209a/slyt209a.pdf

2. “Touch-screen controller tips,” Application Bulletin (LIT# SBAA036) by Skip Osgood, CK Ong, and Rick Downs, April 2000, Texas Instruments: http://focus.ti.com/lit/an/sbaa036/sbaa036.pdf

3. “An Introduction to Mixed-Signal IC Test and Measurement,” Mark Burns and Gordon Roberts, 8 February 2001, TI Sponsored, Oxford University Press: http://www.oup.com/uk/catalogue/?ci=9780195140163

4. For more information about the TSC2005 touch-screen controller, visit:www.ti.com/sc/device/tsc2005

About the Authors

Bonnie Baker is a Senior Applications Engineer for Texas Instruments. She has been involved with analog and digital designs and systems for nearly 20 years. In addition to her fascination with circuit design, Bonnie has a drive to share her knowledge and experience. To that end, Bonnie has written nearly 300 articles, design notes, and application notes, as well as authored the following reference book: “A Baker's Dozen: Real Analog Solutions for Digital Designers.”

Wendy Fang is a Senior Applications Engineer at Texas Instruments. She has been working on electric and electronic fields for nearly 15 years with extensive hardware, firmware and software design and application experience. Wendy holds a BS from Xian Jiao Tong University, Xian, China; an MSEE from the Beijing University of Aeronautics and Astronautics, Beijing, China; and a PhD on Information Technology and Engineering from Gorge Mason University, Fairfax, Virginia.

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