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 todd.severson@cypress.com

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 henry.wong@cypress.com

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