Good strategies for a low-power resistive touchscreen

Engineers are increasingly designing touchscreens into portable devices. Battery technologies are becoming more efficient, but it's still critical to keep the system's power dissipation as low as possible due to the growing sophistication of the electronics around the touchscreen. In addition, the end-user is expecting more time between each recharge of the battery.

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

This article discusses the construction of a resistive touchscreen panel and the inner workings of a typical four-wire resistive screen. It also discusses some hardware low-power strategies.

Four wires
There are various touchscreen technologies. Most of the panel technologies available use resistive, capacitive, SAW or IR techniques. The most popular touchscreen in the market is resistive because it is inherently stable and affordable.

There are four-, five-, seven- and eight-wire resistive touchscreens. The most common resistive touchscreens have four wires. The layers of a four-wire resistive touchscreen panel—from top to bottom—are a rectangular, flexible top layer; a transparent, conductive coated layer (the conductive coating is usually made of 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 four-wire touchscreen panel.

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 so that it depresses, thus allowing 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 touchscreen 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.

Understanding the system
A touchscreen system has a touch panel, a touchscreen controller and a host processor. The touchscreen or touch panel is the "resistive sensor" of the system.

The topology of the touchscreen controller includes a driver for the panel, muxand ADC. The driver in the touchscreen 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 touchscreen controller measures the touch position and pressure by converting the analog voltage from the touchscreen into digital code. Typically, the ADC topology is a successive approximation register (SAR) with resolutions of 8-, 10- or 12bits.

There are two interfaces in the touchscreen system: an analog interface between the panel and the touchscreen controller, and a digital interface between the touchscreen controller and host.

With the four-wire touchscreen panel, use the two active areas of the resistive touchscreens to sense the X and Y pressure points (a). The equivalent circuit is simply a voltage-divider (b).

The touchscreen controller uses the four-wire analog interface between the touch panel and the touchscreen controller to power the panel and execute coordinate measurements. During a given X- or Y-coordinate measurement, the touchscreen controller provides power through two wires (X+ and X-) of the analog interface to one ITO layer of the panel. It also 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 touchscreen controller includes an interrupt signal and a serial digital bus (SPI or I?C). The processor can ignore the touchscreen controller and focus on performing other tasks, if there is no panel touch event. As soon as the panel is touched, the interrupt from the touchscreen controller informs the processor. The processor then reads the touchscreen data from the touchscreen controller through the serial bus.

Analog interface
The primary factors influencing the touch-panel and analog-interface power consumption are the system power supply (V_DD), panel resistance and the panel on:off ratio over time. The driver and ADC topology of the touchscreen 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 touchscreen and the CMOS 12bit SAR ADC is from 1.2V to 5.5V. The power of the touchscreen controller has approximately the same voltage range. Depending on the particular resistive touchscreen panel, the resistance of each ITO layer can range from 100? to several kilo ohms. While the panel is excited, the power dissipation across the panel can range between 302.5mW (power = 5.5V, panel resistance = 100?) to 720?W (power = 1.2V, panel resistance = 2k?). As these calculations show, power-reduction strategies include reducing the power to the panel and touchscreen controller, selecting a higher resistance panel or both.

Because of the high value of power dissipation when panel is turned on, a primary objective for this application is to keep the touch panel turned 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, ESD and/or electromagnetic pulses.

If this type of noisy environment exists, add noise-reduction components in the signal path between the touch panel and the touchscreen controller. Note that any added capacitance in the input line will increase the input settling time for touchscreen controller. An increase in panel settling time decreases the sample rate of the touchscreen controller and increases the power-on duration of the panel drivers.

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. Touchscreen controllers, however, usually run with a much higher sampling rate.

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

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

One might guess 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 400kHz bus speed transmits in just 25 percent of the time the same data transmitted using a 100kHz bus speed. Data transmitted with a 3.4MHz bus speed transmits in just 2.94 percent of the time that the same data takes to transmit with the 100kHz bus speed. Basically, higher bus clocks in the touchscreen system dissipate less average power.

A simple interface between the touchscreen controller and the host immediately sends the coordinate data to the host. With this type of interface, the touchscreen 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 touchscreen controller. The touchscreen 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 to digitally reduce noise in the panel data.

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

Before data transmission, there is minimal communication between the touchscreen controller and the host. Although the rise time of the resistive/touchscreen 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 3 shows three, patent-pending, simple but effective filtering algorithms for a touch panel data set. 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 3b) 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 3a). Another alternative is to throw the low and high values from the data set out and then average the remaining values (Figure 3c). Other debounce algorithms include voting and a processor-implemented FIR filter.

You can derive an averaged (a) or median (b) result from a set of touchscreen 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.

Pre-filtering
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 touchscreen controller for data filtering activity. If you move the filtering activity from the host to the touchscreen controller, you can significantly reduce traffic on the digital bus lines. This change lowers the digital interface power.

It's an advantage when the touchscreen controller senses the touch of the stylus, collects the multiple coordinate data sets, pre-processes the data to reduce system noise and then transmits the final results to the microprocessor. The duration of one I?C clock cycle operating at a 400kHz clock rate is equal to 2.5?s.

Similar traffic reduction happens with a digital SPI. With the SPI, the transmission of one read cycle requires 24 clock cycles. The transmission of seven unfiltered read cycles require 168 clock cycles. A touchscreen controller with built-in filtering features transfers only one set of coordinate data through the SPI bus, 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 or simply an unintentional brushing of the touchscreen. If the touchscreen system responds (with an interrupt) immediately without verification, the touchscreen 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 touchscreen 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.

- Bonnie Baker and Wendy Fang
Senior Applications Engineers
Texas Instruments Inc.