maintaining good user experience as touch screen size increases

http://www.embedded.com/design/power-optimization/4418629/2/Maintaining-good-user-

experience-as-touch-screen-size-increases

Maintaining good user experience as touch

screen size increases

Todd Severson and Henry Wong, Cypress Semiconductor - July 20, 2013

Capacitive touchscreens in consumer electronics to took off with the launch of Apple’s iPhone in

2007. The 3.5” screen introduced a multi-touch user experience that changed the way we interact

with our electronics. Touchscreen displays are now a standard in consumer electronic products

such as DSCs (Digital Still Cameras), PNDs (Portable Navigation Devices), e-readers, tablets,

Ultrabooks and AIO (All-In-One) PCs.

A key trend in all of these devices is the move to larger screen sizes. Not only are capacitive

touchscreens growing to address new market segments such as Ultrabooks or notebooks, they are

also increasing within their current product segment. For example, smartphone OEMs are

making the move from smartphones to superphones, providing larger screen sizes as a key

differentiation in the market.

The main product segments for touch-enabled devices today are smartphones with screen sizes

between 3” to 5”; super-phone or phablet in the range of 5” to 8”; tablets 8” to 11.6;, Ultrabooks

11.6” to 15.6”; and notebooks ranging as high as 17”. Tablets are considered one of the fastest

ramping mobile devices in its five years of product history; sales are predicted to overtake PC

sales by 2015 (Figure 1). This is causing PC vendors to shift their focus to adopting touch-

friendly designs such as convertible notebooks that can function as notebooks or tablets.

Figure 1. Worldwide tablet and PC growth



As screen sizes of touch-enabled devices grow larger, the main challenge for designers is

maintaining the same high performance users have come to expect from a cell phone but over a

larger screen. This means scanning more intersections over more surface area in the same

amount of time. In addition, the processor has to work with less signal and more noise while still

maintaining the speed, precision, and responsiveness required for a desirable user interface

experience.

Users expect large screen devices to have similar performance and touch experience to that of

their smartphones, but large screen devices often deal with different use cases than what is

typical on a smaller phone. Notebooks or PCs are more likely to be used while plugged into a

power source, there is more surface area to rest palms or other large objects on the screen when

typing, and users are more likely to set larger devices on a table or in their lap instead of holding

it in their hands.

All of these conditions and circumstances change the electrical properties of a device. The key

ingredients to a robust and responsive user experience include sensitivity, tracking multiple

moving touch objects, recognizing and tracking fingers in different noise environments,

recognizing and tracking fingers under different environmental conditions, and maintaining

acceptable power consumption to achieve the desired battery life.

Capacitive touchscreens operate by driving a transmit voltage into the sensor panel on the device

that creates a signal charge. This signal is then received by the touchscreen controller, which is

able to determine the sensor capacitance by measuring the change of the sensor charge. The

current received by the chip is equivalent to the capacitance of the panel multiplied by the

voltage of the transmit drive (Q1 = C * VTX). A baseline circuit is able to remove the nominal

non-touch sensor charge so the system can focus on measuring the change of sensor charge due

to finger touch. This improves touch measurement, resolution and sensitivity.

The main problem with larger screens is that the transmit voltage has more surface area to cover

and the resistance and capacitance of the sensor increases. The touch panel is limited by the

higher parasitic capacitance and resistance, affecting the RC time constant, which results in

slower transmit frequency. The transmit operating frequency affects signal settling, refresh rate

and power consumption. The goal is to determine the highest transmit operating frequency

conditions for a consistent touch response across the panels while minimizing scan time and

power.

Refresh rates versus user interface needs Refresh rate is the number of times in a second that the touchscreen controller can measure a

touch on the screen and report it back to the host processor. A higher refresh rate will provide a

responsive user experience by collecting more x/y data coordinates in a shorter amount of time.

Most consumer electronics devices require a touch controller refresh rate of greater than 100 Hz,

or about 10 ms. Certain applications, such as digital drawing pads or Point of Sale (POS)

terminals require even higher refresh rates to capture and recognize signatures and quick pen

strokes.



It is challenging for large screens to maintain fast refresh rates because the touch controller needs

to sweep greater surface area, gather data from all the intersections, and then process that data.

The two main components that effect refresh rate are how fast the screen is scanned and how fast

the scanned data is processed. A 17” screen has 11 times more intersections than a 5” screen with

the same sensor characteristics (3108 vs. 275). In order to maintain the user experience of the 5”

screen, the 17” screen requires more scanning and processing power.

One technique to help solve the scanning problem is to make sure the touch controller has

enough receive channels to sweep the screen in a single pass. Most touchscreen stack-ups are

composed of sensor patterns under the cover glass in an array of ‘unit cells’ that run in the x and

y direction, with x being transmit and y being receive or vice versa. The receive channel will

collect the data and use analog to digital converters (ADC) to convert the change in mutual

capacitance of each unit cell into digital data for the host to interpret where the finger touch

coordinates are located. If the number of receive channels or ADCs are inadequate, then it will

take multiple scans and more time to sweep the entire panel. This results in fewer samples that

can be taken in a given time period, leading to an unsatisfactory user experience.

A technique to help solve the processing problem is to add a bigger processor to the touch

controller or offload some of the computing to the system’s main processing unit. This means

sending capacitive data to the host side and running algorithms on the applications or graphics

processor. One implementation would be to use the touchscreen controller to scan the sensor,

search for first touch, and then transfer the image to the host processor. The host will then

process the full array, filter noise, find touch coordinates and track finger IDs. This use of

parallel processing allows the heavy number crunching to be done in the multi-GHz, multi-core

processors that serve as a host for the touchscreen and display.

Changing requirements for panel SNR SNR (signal to noise ratio) is the ratio of signal power to noise power, or, in other words, the

ratio of useful information to false or irrelevant data. The sensor on a touchscreen panel acts as a

large antenna (Figure 2) that is able to pick up system and environmental noise such as

fluorescent lights, LCDs or chargers.

Figure 2. Flatpanel screens act like antennae for noise signals



Larger screens act as larger antennas so it is easier to pick up noise and saturate a receive

channel. This can greatly affect touch performance by causing false touches, dropped touches, or

a locked up touchscreen that will not report data at all. In order to overcome this interference, the

touchscreen controller needs to be able to increase signal or decrease noise. Some of the primary

ways to achieve better SNR include boosting the transmit voltage to increase signal, using

hardware and digital filtering to decrease noise, or using frequency hopping to move away from

noisy frequencies.

SNR increases linearly, proportional with transmit voltage. Transmit voltage can be delivered

from a transmit charge pump or VDDA driver. A charge pump is able to take a typical 2.7-3V

power supply, found in most consumer electronic devices, and boost it up to a higher voltage.

The problem with large screens is that a charge pump has limited drive strength capability for

high capacitance panels. This means that an external pump or power supply must be added,

which can increase cost and power consumption.

If there is not enough signal, the other option is to minimize noise. The first line of defense is

using filters to create a cleaner capacitive image. If this is not effective the second line of defense

is using frequency hopping to find a frequency where there is less interference.

As mentioned earlier, large panels have higher parasitic capacitance and resistance, affecting the

RC time constant that results in a slower transmit frequency. A slower frequency means it is

harder to scan the panel outside of the noise range. A higher transmit frequency gives the touch

controller more room to move away from a noise source. A max transmit frequency of 350 kHz

or greater is ideal, but a constant trade-off between SNR, refresh rate and power is required to

optimize each device based on the customer’s objectives. An individual playing games on a

desktop PC is more interested in responsiveness than power consumption, whereas portable

devices need to account for power consumption to save on battery life.

Bigger screens and power consumption As mobility becomes a bigger part of our lives, power consumption is a key factor in a

consumer’s selection for portable electronic devices. Market surveys (Figure 3) show that a

majority of users believe battery life is one of the most important features when purchasing a

new portable device.



Figure 3. Users want bigger screens AND longer battery life.

The LCD is a big portion of the power draw from the overall system. Power usually scales with

larger screens due to the increased LCD size. One way of maintaining battery life is to put a

larger battery pack in the system. However, this increases the weight of the system and affects

the user experience in terms of portability. Another alternative is to decrease performance by

reducing refresh rate, reducing transmit voltage, disabling various digital filters, or using the

lowest possible analog and digital power supplies. Again, these solutions negatively impact the

user experience so they are not ideal options.

As weight and performance are key factors to a good device, the best resolution for extending

battery life is to optimize power draw for individual components in the system. From a

touchscreen controller point of view, that means having flexible power management schemes for

the device.

The overall power consumption depends on the state or usage of the device (Figure 4). A smart

and energy efficient touchscreen controller has multi-state power management in which each

state has a unique scheme to lower power consumption, such as an active state, low power state,

and deep sleep state. This is all managed by the touch controller’s configuration parameters.

The active state provides the fastest touch response time because the touchscreen is

actively scanned to determine the presence of a touch and identify the coordinates.

The low power state is entered when no touch is detected after a certain time during the

active state. This state further reduces power with corresponding increase in the response

time. Any touch detected will automatically switch the device into active state.



The deep sleep state has the lowest power consumption. No scanning is performed and no

touches are reported. An interrupt is required to wake up the touch screen controller and

put it into active state.

Figure 4. Power useage depends on LCD UI configuration state.

The various power states are determined by the system environment. For example, if the screen

hasn’t been touched in a while, the system will deactivate the user interface to save battery life.

This is done by the host managing the components in the device, for example by turning off the

LCD screen and placing the touch controller into a low-power state. When a touch is detected in

the low-power state, the touchscreen controller will transition to active mode and continue

scanning to determine the touch coordinates on the panel. If no touch is detected in the low-

power mode, the host will drive the touch controller into deep sleep to conserve power. These

dynamic power management states provide consumers flexibility between touch performance

and power consumption for mobile devices on-the-go.

Maintaining satisfactory user experience as touchscreens grow takes a system wide approach.

Touchscreens are limited by physics, and if capacitive touch is to remain the technology of

choice in mobile consumer electronic devices, then ingenuity and integration are key. New

touchscreen materials are being developed to increase panel speeds, and host processing

architectures are being defined to offload some of the heavy number crunching. Hardware and

software improvements are constantly being made to increase signal strength while filtering out

noise. A system wide approach to power consumption is being used to increase battery life.

Making this all more cost effective is the next big challenge for designers.

Todd Severson is a Product Marketing Engineer for TrueTouch touchscreen solutions at

Cypress Semiconductor Corp. He has a BS degree in Engineering Management with a

concentration in Mechanical Engineering from the United States Military Academy. You may

reach him at [email protected]

mailto:[email protected]




Henry Wong is a Senior Product Marketing Manager for TrueTouch touchscreen solutions at

Cypress. He has a BS degree in Computer and Systems Engineering from Rensselaer Polytechnic

Institute. Henry has over 16 years of engineering and marketing experience in the semiconductor

and consumer electronics industry worldwide. You may reach him at [email protected]


maintaining good user experience as touch screen size increases

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