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Human-to-Machine Interface (HMI) touch sense display technology exists in a variety of forms, such as resistive, capacitive, infrared, and acoustic.  In industrial applications, the resistive touch sense market has the broadest share due to simplicity, resolution, and lower cost.  Resistive touch sensing is mechanically actuated by conductive contact that can be activated by a pencil eraser, stylus, fingernail, or gloved finger.  However, resistive touchscreens are prone to mechanical wear out, are adversely affected by dust or water, are susceptible to scratches from sharp objects, and have user problems with optical parallax quality.

Capacitive Sense technology is gaining popularity in industrial designs.  The system-level reliability and lower design costs are driving capacitive touch sense adoption into many industrial control and home automation operator interface designs. Capacitive sense is replacing buttons, rotary and slider switches featuring improved system reliability over electro-mechanical components.  Most capacitive touchscreen sensors consist of two transparent conductive layers that are separated by two layers that provide superior optical quality, highly durable, long-life reliability, and multi-touch sensing. 

Design Issues

Capacitive sensor applications use components to measure minute analog changes and post-sample to interpret the measure change to characterise operate touch control. Designers must overcome barriers to achieve a robust operator interface control solution:

  • Balances touch sensitivity verses measurement current consumption.
  • Optimise sensor layout, stack-up and system-level considerations
  • Creates firmware that filters interference, false touches, and produces reliable touch impression data

In industrial applications, environmental noise can cause misinterpreting of user touch or human proximity resulting in poor user experience.  A higher signal-to-noise ratio (SNR) is desirable to detect the presence of a user’s touch while being immune to noise sources.  Response time of interpreting a touch event from the user’s interaction to the control screen to the confirmation of user’s designer action is a desired operational function. Power consumption of capacitive touch sense interface is directly related to the sensitivity.  Designers must determine the programmable thresholds and current consumptions limits to optimise the optimise touch control sensitivity, reliability and current consumption. 

In summary, here are some recommendations to limit capacitive sensor design problems:

  • Utilise vendors sensor measurement sample code
  • Stress test sensor measurement robustness in industrial and noisy environments
  • Tune and balance sensitivity and firmware to optimise power management budget.

 

Developer’s approach to adding Capacitive Sense

Designers looking to add capacitive sense to their products need to consider use of a dedicated device or a general purpose MCU with integrated capacitive sense functionality.  Each solution provides benefits depending on target product, price point, product size, and time to market.  Capacitive sense solutions are available in two forms:  MCU-based and dedicated fixed-function.

  • MCU-based - requires firmware development
  • Fixed-Function - does not require any coding development

 

MCU-based Capacitive Sensing

A MCU-based, integrated capacitive sense solution is ideal when multiple responsibilities can be executed from one MCU when space is constrained, the sensor must use a serial or parallel interface to communicate in the system, and the system specification is beyond the scope of a fixed-function device. An integrated MCU-based capacitive sense solution must perform multiple system functions that requires MCU peripheral or memory configuration beyond the available capacitive sense MCU features.  Integrated MCU-based capacitive sense challenges include coding expertise and resource constraints.

Dedicated MCU-based deployment for capacitive sense tasks requires shared processing time to perform system-level and housekeeping functions.  Performance scenarios must balance among MCU tasks.  Integration and testing of capacitive sensing sample time and sensitivity tuning will require higher power operation for the sensing function that will degrade product power consumption. 

Experienced In-House coding expertise in development capacitive touch sense firmware is a development concern.  Choosing an MCU from a vendor that has comprehensive capacitive sensing firmware and software support is essential.  Even if vendor source code capacitive sensor scanning and measure routines exist there are design code testing and integration challenges.

Fixed-Function Capacitive Sensing

Fixed-function capacitive sense devices eliminates the challenges of developing firmware while optimising touch-sense performance.   The fixed-function device requires a host MCU to monitor the capacitive sensing events.  The fixed-function device can perform the monitoring function in a low power polling mode while the host MCU is in a deep-sleep mode conserving system power.

Silicon Labs created an end-to-end fixed-function capacitive sensing solution that helps eliminate these challenges and provides:

  • Reliable touch qualification
  • Optimised current consumption
  • Easy configuration through sophisticated software tools; eliminates complex firmware development.

 

Comparison table of capacitive sense devices:  MCU-based and fixed-function.

Design Stage

MCU-based

Fixed-Function

Prototyping

Leaning ramp to get sense code running

No coding required; vendor capacitive sensing eval kit can be used to optimize touch sensitivity

Board layout

Flexible for uncommon use cases

Defined best practice layout recommendations speed development

Performance tuning

Code debugging, algorithm development and refinement mean longer development time

No coding required, performance tuned by vendor software tools

System integration

Programmable MCUs provide higher functional density when designs benefit from giving one MCU multiple responsibilities

Limited to the capacitive sensing functions on device

   

Interference and moisture immunity

Developer responsible for coding and testing, aided by support vendor provides with documentation and firmware

Can rely on vendor’s fixed-function device testing and specifications

 

Silicon Labs Capacitive Sense Solutions

Silicon Labs developed a capacitive sense technology called “charged timing” that uses an external capacitive plate that’s charged and discharged periodically, then compared to a reference capacitance value.  This charge timing technique is power efficient and generates a very high signal-to-noise (SNR). The charged timings technique can detect very minute changes in capacitance at low power levels.  This technology is incorporated in all Silicon Labs capacitive sense measurement solutions.

Silicon Labs’ 8- and 32-bit capacitive touch sense microcontrollers (MCUs) enable implementation of touch buttons, sliders, wheels, capacitive proximity sensing and liquid level sensing.  These MCUs offer relaxation-oscillator (RO) technology and the industry’s fastest charge-timing Capacitance to Digital Converter (CDC), for capacitive touch sense measurement applications.

Silicon Labs EFM8SB Sleepy Bee Microcontroller (MCU) family offers capacitive sense functionality as an autonomous MCU peripheral.  The capacitive sense peripheral runs without interrupting or waking the EFM8SB MCU core or peripheral that is ideal for low-power, battery operated designs.  The EFM8SB 8-bit CIP-51 MCU core offers up to 14 capacitive sense channels , a small single-device footprint, which offers a cost advantage over a two-chip MCU plus separate capacitive sensor controller architecture.

Silicon Labs CPT007 and CPT112S TouchXpress fixed-function devices give developers a streamlined path toward adding button and slider capacitive inputs to a design.  Developers can utilise the Silicon Labs CPT evaluation kits for proof-of-concept phase testing. These kits have sensor designs onboard that simulate buttons in the final product. 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.

Vendors provide reference designs, software libraries, evaluation boards, and configuration tools, which are intended to reduce design complexity. These support tools are utilised at different stages of the development process. Heres the design stages for a capacitive sensing product development flow:

  • Evaluation — usually involves using vendor-provided evaluation boards.
  • Fly-wire prototyping — In this stage, the developer may route wires from a vendor evaluation kit to copper tape in a product mock-up
  • Case to simulate the touch interface.
  • In-system prototyping — This stage involves low volume PCB manufacturing with capacitive sense devices on-board, routed to
  • PCB-implemented touch sensors.
  • Deployment —the final configuration of the capacitive sensing device is installed in the final revision of the product hardware.

 Silicon Labs Simplicity Studio

Simplicity Studio 4.0 simplifies the MCU development process with one-click access to everything developers need to complete their projects using an integrated development environment (IDE) based on Eclipse 4.5. Simplicity Studio includes a powerful suite of tools for energy profiling, configuration and wireless network analysis, as well as demos, software examples, complete documentation, technical support.  Simplicity Studio provides built-in intelligence to automatically detect the connected 8-bit or 32-bit MCU or wireless SoC, graphically configure the device, and show supported configuration options to help developers get their projects underway in minutes. The tools provided by Simplicity Studio enable the development flow for the Silicon Labs capacitive sense peripheral MCUs and TouchXpress devices.  Simplicity Studio Tools for Capacitive Sensing include:

  • XPress Configurator: easily configure characteristics of each sensor
  • Capacitive Sense Profiler: properly examine real-time capacitive output

Xpress configurator allows designers to customise activation thresholds, gain, active mode and sleep mode scan periods, operating frequency, and drive strength.  This figure illustrates Xpress configurator GUI tool.

'Example of Xpress Configurator Interface for TouchXpress Customization

Capacitive Sense Profiler allows designers to interface the TouchXpress Evaluation Board to the design sensors directly to visualise the capacitive device’s performance and output information such as detection time, touch thresholds, charge levels, etc.  This figure shows the Sense Profiler touch screen waveform and parametric capture feature.

Example of Capacitive Sense Profiler Interface

The TouchXpress devices, combined with tools provided in Simplicity Studio, support each step of the development process with both configuration and real-time capacitive sensing data visualisation. Touch sensing development usually falls into an iterative workflow, where a configuration is defined, tested, refined, and tested further. The test loop completes when a configuration achieves the responsiveness and current draw requirements of the design.

At the prototype stage, designers can use Simplicity Studio to optimise and improve a CPT007 or CPT012S device in-system through a two-wire interface that can be routed to pads or to 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 affects 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.

 Conclusion

The cost and design complexity with integrating capacitive touch sensing for replacing buttons, rotary and slider switches is being reduced.  Capacitive sense peripheral MCUs combined with sensor measurement configuration software tools have simplified sensor design integration and testing.  Fixed-function capacitive touch sense measurement devices eliminate the firmware development effort by providing sensor measurement configurator GUI tools.  The Silicon Labs CPT sensing devices offer a combination of robust hardware and configuration software that can expedite capacitive sensor design time while overcoming the technology design pitfalls.

Featured products (865-6294) (865-6301) (865-6305) (865-6308) (865-6323)

Silicon Labs Contributing Authors:

Mark Beecham, MCU Applications Engineer

Parker Dorris, MCU Applications Engineer

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