Not All Touchscreens are Created Equal - How to Ensure You Are Developing a
World Class Touch Product
By Steve Kolokowsky, Senior MTS Elect Design Engineer and Trevor Davis, Marketing Director, Cypress Semiconductor
Corp.
Anyone can bake a cake – but while some chefs bake dry, uninspired bricks of dough, there are other chefs who make cakes
we would die for. The ingredients may be the same, but the outcomes are so very different. This is also the case between
average electronic products and world-class market-changing products. One of the most recent technical sensations is the
capacitive touchscreen. But what makes some touchscreens amazing, while others get such poor reviews? This article
explores key touchscreen performance parameters, critical touchscreen design features, significant design tradeoffs, and key
issues product designers must consider when choosing their touchscreen supply chain. Don’t get caught making an uninspired
product – create something people would die to have.
Perhaps the single most significant technology change to affect the performance of today’s touchscreens has been the shift
from resistive to capacitive touchscreens. Industry analyst iSupply forecasts that nearly 25% of the mobile handsets with
touchscreens will have shifted from resistive to capacitive screens by 2011 and Jeffries and Co. has increased their
projections for capacitive touchscreens in 2010 from 100Mu to 188Mu. The market is exploding in large part because of the
benefits capacitive touchscreen technology brings.
While traditional resistive touchpanels detect a finger or stylus touch when a flexible top layer of clear material is pressed down
to contact a lower conductive layer of material, projected capacitance screens, conversely, have no moving parts. In fact, the
projected capacitance sensing hardware consists of a glass top layer, followed by an array of X, Y, and insulating layers of
Indium Tin Oxide (ITO) on a glass substrate. Some sensor suppliers create a single-layer sensor that includes both X and Y
sensors in a single layer of ITO with small bridges where they cross. As a finger or other conductive object approaches the
screen, it creates a capacitor between the sensors and the finger. This capacitor is small relative to the others in the system
(about 0.5pF out of 20pF), but it is measurable by several techniques. One technique used with Cypress Semiconductor’s
TrueTouch parts involves rapidly charging the capacitor and measuring the discharge time through a bleed resistor.
This all-glass touch surface gives the user a solid, smooth feel across the entire screen. Glass screens are preferred by
customers because glass gives the end product a smooth industrial design, and provides a good capacitive signal for
measuring touch. In the end, however, it is not only how the touchpanel looks, but rather how it operates. And gaining
excellent performance from a touchscreen entails first knowing several key parameters.
Accuracy – Defined as “the maximum position error across a pre-defined area of the touchscreen, as measured in units of
distance along a straight line between the actual finger position and the reported finger position.”
Accuracy is measured with a simulated or mechanical finger. The finger is placed at precise locations on the panel and actual
finger position is compared with reported finger position. The importance of accuracy cannot be overstated. Users want the
system to properly locate their finger. One of the biggest frustrations with resistive touchscreens is their low accuracy and
accuracy loss over time. Capacitive touchscreens’ accuracy enables new applications like virtual keyboards and handwriting
without a stylus. As an example, Figure 1 shows a poorly constructed touchpanel’s data that displays finger drift above the true
straight line traveled by the simulated finger.
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FIGURE 1: Example Showing Inaccuracy or error in a Touchpanel tracking
Finger Separation – defined as “The minimum center to center distance between two fingers placed on the touchscreen while
two separate fingers are still reported by the touchscreen controller.” Finger separation is measured by placing 2 simulated or
mechanical fingers on the panel and moving them towards each other until they are reported as a single finger. Some
touchscreen suppliers report finger separation as edge-to-edge, while others report it center to center. A 10mm finger
separation spec for 10mm mechanical fingers could mean that the fingers are touching or that they are 10mm apart depending
on your touchcontroller’s method of specification.
Without good finger separation, you don’t really have a multi-touch solution. Finger separation is especially important for
virtual keyboards, where two fingers are commonly on the screen in close proximity.
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FIGURE 2: Measuring Finger Separation
Response Time – Defined as “The time between a finger touchdown event on the touchscreen and the touchscreen controller
generating an interrupt signal.” This can be measured by electrically stimulating the touchscreen to simulate a finger or by
physically moving a simulated finger to the panel. Response time is particularly important because it directly translates to how
fast a user can move their fingers on screen for a “swipe” or “flick” or to write with a finger or pen. A touchpanel with a slow
response time may look choppy or may miss a movement altogether. Touchscreen response time is one component of system
response time that includes
-
X/Y scanning: Time for the touch controller to scan and measure the change in capacitance on the sensor.
Finger detection: Comparing the capacitance change on the panel to a predetermined “finger threshold”. If the change
is over the finger threshold, a finger has been detected.
Finger location: Interpolating between the results from several sensors to determine the exact position of the finger.
Finger tracking: When more than one finger is on the sensor, each finger must be identified and assigned a unique
identifier.
Interrupt latency: This is the delay between the interrupt indication and interrupt servicing on the host. In most systems
this delay is less than 100uSec.
Communication: Typical systems use I2c at 400 KHz or SPI at 1MHz to communicate to the host.
There are several tools that can be used to reduce response time. The key is primarily in the intelligence of the touch
controller IC. For example, creative techniques can be employed for only scanning portions of a screen to “detect” a finger and
once a finger is detected, to then rapidly scan to calculate exact “location” of that finger thus saving both power and time.
Parallelism is another key tool. Scanning, finger processing, and communication all use separate hardware and can all run in
parallel. Highly optimized algorithms for finger detection, finger location and finger ID all reduce processing time and reduce
response time.
Refresh Rate - The time between two consecutive frames of touchscreen data available in a data buffer while a finger is
present on the touchscreen. A low refresh rates will result in jerky movement and curves that appear to be made up of line
segments rather than smooth curves. Instead, if a touchpanel has a high refresh rate, it provides many more datapoints for
interpretation of a smooth or complete shape or motion. A high refresh rate improves gesture interpretation as well. Smart
touchscreen controllers like Cypress Semiconductor’s TrueTouch™ products can adjust their refresh rate to match the
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system’s requirements. A drawing or handwriting application needs a fast refresh rate, but a mobile phone dialing keypad only
needs to interrupt the host when buttons are pressed or released.
Average Power Consumption - The average power in a touch system is comprised of time scanning, time processing, time
communicating, and time sleeping for the controller IC and host processor receiving and interpreting touch data.
Power consumption seems like an obvious performance parameter: Measure the current used by the device, multiply by
voltage and you know the power consumption. In the world of touchpanel power consumption, however, a more sophisticated
model is needed because power consumption depends on usage. Phone “standby time” will depend on touchscreen
“standby” or “deep sleep” current use. Even when the touchscreen is active, it can be in several modes like “wake on touch”
(WOT), “touch”, and “cheek detect”. In a typical 5 minute call, your phone may be in touch mode for 10 seconds as you lookup
or type in the phone number, then it will be in WOT or cheek detect for the remainder of the call. Even sending a text (SMS)
message is a mixture of WOT mode and actual finger contact as the controller IC dips in and out of sleep modes while you are
typing and thinking.
It’s easy to be misled by system power promises if you don’t take these power modes into account. In almost every case, the
touchscreen spends 90-99% of the time in “cheek detect” mode and “wake on touch” mode. Some systems allow
customization of the ratio of processing time to sleep time even while a finger is on the panel. There is little need for a 200Hz
refresh rate if the system is only reporting “there is still a finger at the same location”. To develop a high performing
touchscreen, it is important to take advantage of systems with low sleep specifications along with creative sleep and wake
modes.
There are several other important parameters that system designers must keep in mind when designing a capacitive
touchscreen system.
Finger Capacitance – The capacitance measured between a finger and a single sensor element. Finger capacitance is
measured using a real finger rather than a metal finger to ensure real-world data. Factors that affect CF include cover lens
thickness and the dialectric constant of the cover lens material.
System Noise Floor - The amount of noise measured at the output of the capacitance-to-digital-converter referred to the input
(capacitance) of the data converter.
Signal to Noise Ratio (SNR) - The finger signal measured on one sensor divided by the observed measurement noise. While
this is an important shorthand for the two measurements above, it must be understood that to create an effective touchpanel,
the system must be able to accommodate, adapt, and filter parasitic noise in a mobile system. In order to observe high signal
counts and very low noise counts, an accurate analog front end should be considered for the touch function.
FIGURE 3: Signal to Noise (SNR) Example
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Programmable solutions like Cypress Semiconductor’s TrueTouch™ family provide excellent mechanisms for filtering noise.
The PSoC® programmable analog can be reconfigured to integrate signals over longer time periods to filter noise. Different
signaling frequencies, including spread spectrum and pseudo-random frequencies can be used to avoid EMI. Standard digital
filters can remove 1-2 bit signal jitter or provide a low-pass filter like an IIR. Smart digital filters can discard samples that don’t
“look right” compared to the samples near them on the panel. Smart filters are limited only by the system designer’s ingenuity.
Figure 3 shows an example pattern of device noise floor and a registered touch. In this case, a SNR of 5 is registered.
Understanding and controlling key touchscreen performance metrics will lead to a significantly better performing touchscreen
design. Understanding these metrics also allows you to select design partners who have technology that can take advantage
of the unknown noise and electrical issues in a mobile consumer product.
The beauty of touchscreens is their seemingly simple design. Void of clunky buttons, sticky roller balls, or barely-readable
screens, touchscreens are an entirely new way of creating an enjoyable user experience. The difficulty with touchscreens,
however, is that to be able to deliver an elegant, simple design, you must use sophisticated hardware, firmware, and
manufacturing techniques. Understanding the language of touchscreens, key design parameters, key touchscreen
performance parameters, and touchscreen design tradeoffs is the first step in building a market inflecting touchscreen product
– we look forward to touching your product soon!
Cypress Semiconductor
198 Champion Court
San Jose, CA 95134-1709
Phone: 408-943-2600
Fax: 408-943-4730
http://www.cypress.com
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