The C8051F7​xx family of high pin-count capacitive touch sense microcontroller (MCU) devices for cost-sensitive, high I/O embedded systems offer a patent-pending touch sensing feature that is robust, accurate, responsive and easy to configure.

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Overcome the Top 4
Challenges of Capacitive
Sense Design
By Parker Dorris, Applications Engineer,
Silicon Labs

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1
Introduction
From light switches to washing machines, developers are adding capacitive sense to hundreds of devices that
need a sleek, low-­profile interface surface, improved long-­term reliability compared to mechanical buttons, and a
customized interface.
All capacitive sensing solutions essentially perform the same task. Sensors use integrated CMOS designs
combined with external capacitive components to measure minute analog changes. Then, post-­sample
processing interprets these changes to characterize user touches.
Although capacitive sensing solutions have been on the market for decades, this technology still has formidable
design barriers, especially in early-­stage development.
Engineers must do the following to create an optimal solution:
• Balance the often-­conflicting priorities of touch robustness and current consumption
• Focus on sensor layout, stack-­up and other system-­level considerations
• Create firmware that filters interference, qualifies touches, and produces reliable touch data
Additionally, any sensitive analog sub-­system in a mixed-­signal design faces the challenge of performing
optimally in systems where almost every factor in a system (ie. current draw, timing) work in concert to degrade
performance.
In order to minimize these complexities and streamline the development process, developers must determine if an
integrated approach is better than a fixed-­function one for their application.
Integrated vs. Fixed Function Approach
The numerous capacitive sensing solutions on the market today can be divided into two camps:
• those that require some firmware development
• and those that do not require any coding.
Before developers begin evaluating solutions, they must determine which type of solution is right for their product.
Option A: Integrated Solution
An MCU-­based, integrated solution is beneficial when multiple responsibilities need to come from one
microcontroller because
• the device has space constraints

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• the sensor must use a less-­common serial or parallel interface to communicate to the larger system
• system level requirements fall outside the capabilities of fixed function solutions
However, this isn’t a foolproof option. Integrating capacitive sensing into a central MCU creates additional
challenges, including coding issues and resource constraints.
1. In-­house coding expertise. The worst case is when a developer has chosen an MCU from a vendor that
doesn’t have comprehensive capacitive sensing firmware and software support;; therefore, all sensing firmware
must be developed in-­house. Coding, debugging, and finally optimizing capacitive sensing firmware against all of
the factors that can degrade performance in the field is a task that companies easily devote entire teams to
deliver. Ideally, developers will find a way to avoid this treacherous path and instead choose a vendor that
provides firmware to be used as a starting point. However, even in the case where an MCU is running thoroughly
tested code, integration challenges remain.
2. MCU resource constraints. In an MCU that is responsible for multiple tasks in a system, capacitive sensing
sample and processing time must be shared among all other responsibilities. Resource sharing requires careful
examination of expected worst-­case runtime scenarios so that a balance can be struck among components;;
otherwise, the capacitive sensing component could become a 'starved thread' that isn't responsive or reliable
enough to be product-­ready. Current consumption is another aspect of resource sharing that must be taken into
consideration. MCUs chosen to perform many tasks are often relatively power-­hungry. Without careful tuning of
capacitive sensing sample time, such an MCU could be forced into a higher power state for a percentage of time
that blows through a product's current budget.
Option B: Fixed-­Function Solution
Fixed-­function capacitive solutions eliminate the challenges presented by the integrated approach. This option
frees them from the low-­level coding concerns of touch qualification and nuanced state maintenance. It also
gives them more development cycles to hone the product features and optimize other system-­level concerns.
Since all of the touch qualification and interference filtering techniques are baked into the fixed function device,
developers avoid costly firmware development. Instead of managing all the close-­to-­the-­metal details of
conversion and processing, the developer will be presented with an interface that is geared toward providing
information as concisely and promptly as possible.
A fixed-­function device requires that a host MCU be present in a system, monitoring the device for capacitive
sensing events and other responsibilities. At first glance, this might seem to present a current draw issue

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compared to a solution using the integrated approach;; however, since these devices are optimized to perform a
narrowly defined task, they use minimal current draw.
Simultaneously, these devices are usually designed to signal a host MCU in an event-­driven manner, meaning
they don't require constant polling from a host. This use-­model enables a host MCU to remain in its lowest power
state for a higher percentage of time while monitoring the fixed-­function, signaling pin as a wake-­up source. In
cases such as these, a two-­chip solution results in lower system current draw. The larger chip remains in deep
sleep mode while the smaller, more current-­conscientious chip efficiently scans capacitive sensing inputs.
Real World Example: Remote Control
Either approach often works;; however, developers should examine how consumers will use their product to
determine which option is best. For example, let’s look at remote control devices, which are increasingly using
capacitive sensing surfaces for user input. In its simplest form, a remote control only has a few components: a
user input system, IR transmitter circuit, and an MCU to control the system. Because the product requirements
are not resource-­intensive, an integrated approach might seem ideal to help eliminate the resource-­sharing
challenges between the capacitive sensing and the IR output components.
A closer examination of this use case reveals how a fixed-­function device may lead to a more optimized, energy-­
efficient system. Because all remote control behavior begins with user input -­ specifically a user pressing one of

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the buttons on the remote -­ the MCU controlling other components of the system (ie. IR output) can be placed in a
deep sleep to minimize current draw and increase battery life.
Also, because the system requirements of the MCU are likely not stringent in this type of application, using an
optimized, fixed-­function solution means that the controlling MCU needs fewer port pins to operate, because it will
not need to sense each input on its own. A lower pin count MCU is likely a lower-­cost MCU, potentially enabling a
developer to design in an MCU that is 'just good enough' rather than a chip that is more functionally dense and
geared toward 'system on a chip'-­style use.
Solution to Capacitive Sensing Design Challenges
Let’s continue to focus on the remote control example. While capacitive sensing poses some challenges
throughout the entire development cycle of this application, most issues occur because of decisions made during
the proof-­of-­concept stage.
Here are the top four early-­stage design challenges:
1. Rely heavily on sample code. Vendors frequently provide capacitive sensing-­enabled code examples that run
well in demo environments and during proofs-­of-­concept;; however, when dropped into product firmware, the
sample code lacks production-­ready features. This misstep slows progress and creates panicked and expensive
revisions, incorporating sub-­optimal hacks to get the device released on time.
2. Test too late. One rule of thumb is that a design never works better than it does when it’s on the developer’s
desk. In many ways, a workstation can be an ideal environment for capacitive sensing because so many
environmental variables are carefully controlled. Stress testing later in development often reveals susceptibility to
interference, which can be an insidious problem and hard to characterize.
3. Turn sensitivity and robustness firmware to the max. In many cases, developers begin work on an isolated
system such as a vendor evaluation board. A tendency to ‘turn every knob’ toward sensitivity and robustness in
early firmware without noticing that in almost every case improving those metrics comes at the cost of increased
current draw and sensing time.
As with most analog measurements, capacitive-­sensing touch qualification benefits from sampling more data,
more frequently. Unfortunately, sampling at higher frequencies tends to put capacitive sensors into states where
current draw is relatively high. Additionally, turning robustness-­related features to their maximum can cause
touch qualification response time to decrease. The impact of this decreased responsiveness is not fully apparent
at first because the system in the proof-­of-­concept stage might still be responsible for only monitoring for touches.

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In later-­stage development when the microcontroller (MCU) cycles are shared between capacitive sensing and
other responsibilities, the decreased responsiveness can be exacerbated.
4. Experience sensor design complexity. Sensor design challenges cause multiple prototype builds and
schedule delays. The shape of a capacitive sense electrode and its proximity to ground pour on a PCB or other
board components can lead to interference problems and other issues. Because in many cases the developer's
first tests of a capacitive sensing solution are designed on vendor evaluation boards, they are unencumbered by
design constraints of a developer's final product. When the solution moves to a board with a different layout,
sensing performance may degrade.
Silicon Labs created an end-­to-­end fixed-­function capacitive sensing solution that helps eliminate these
challenges and provides the following:
• Reliable touch qualification
• Best-­in-­class current consumption
• Easy configuration through sophisticated software tools
Our CPT007 and CPT112S fixed-­function devices give developers a streamlined path toward adding button and
slider capacitive inputs to a design. If a developer chooses a Silicon Labs CPT device, they can enter the proof-­
of-­concept phase using the evaluation kits. These kits have sensor designs onboard that simulate buttons in the
final product.
Additionally, the CPT device interface can be wired to a host MCU, while the CPT device's configuration and data
interface can connect to Simplicity Studio, an integrated software development environment.
This software includes specialized items, including the following:
• XPress Configurator: easily configure characteristics of each sensor
• Capacitive Sense Profiler: properly examine real-­time capacitive output
Once developers design a prototype board, they can use Simplicity Studio to optimize and improve a CPT device
in-­system through a two-­wire interface that can be routed to pads or vias on the prototype board. This feature
helps developers overcome challenges that occur when first moving a system away from 'optimal' proof-­of-­
concept designs using vendor evaluation boards. Developers can prototype a product's enclosure and different
types of overlays without losing critical visibility into how each design choice effects capacitive sensing
performance. Furthermore, our documentation guiding sensor design ensures that a developer designs a board
that performs optimally, even in a tightly integrated system.

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Conclusion
Focusing on the minutia of a product's capacitive sensing feature is a waste of time. The more effort poured into
tweaking firmware or spinning new boards to avoid crippling interference coupling, the less time you can devote to
making innovative products. The Silicon Labs CPT sensing devices offer a combination of robust hardware and
supporting software that can help you overcome the many pitfalls of capacitive sensing design, clearing a path for
product design creativity.
About the Author
Parker Dorris is a senior applications engineer supporting the Silicon Labs microcontroller product line. He
specializes in the areas of human interface and USB embedded system design and holds a BSEE from The
University of Texas at Austin.