by Mark Beecham
Adding Capacitive Sense Screens to Just About Everything
From light switches to washing machines, developers are adding capacitive sense controls to hundreds of devices that benefit from improved long-term reliability and a sleek, low-profile, customized interface. But the capacitive sensing technology behind this growing trend can be a design challenge for even senior developers. So why add it?
Why Add Touch Sense?
Touch sense screens are sexy, but they're more than just a pretty interface.Because there are no moving buttons, rockers, or knobs in a touch sense screen there are no moving parts to break, and therefore there are lower maintenance costs. This increased reliability and lower cost drives almost instant benefits since maintenance calls can be extremely expensive. This is pushing touch control into many devices, from white goods to industrial controls to home automation.
Additionaly, the cost for touch sensing solutions has decreased over time as the underlying technology has become far more cost effective due to the enormous volumes of the touch-screen handset revolution. What was once a high-priced oddity in the marketplace and perceived as "very high end," is now commonplace and very inexpensive to manufacture.
Furthermore, technology innovation has driven costs down. Touch sense solutions have gone from dedicated modules with dozens of components to single CMOS-based ICs that are connected to low-cost touch screens, printed on plastic, or even metal, as part of the overall device enclosure. This innovation has not only driven cost down, it has also decreased complexity in touch design, making it easier to add to almost any device.
Example Microwave Oven with Capacitive Sense Controls in the Casing
Mechanical buttons and sliders are passé by today's standards. Users want to buy the state-of-the-art devices with a touch control interface, and as a result of cost reductions and innovation, manufacturers want to provide them.
Capacitive Sensing Basics
Most of today's interfaces work on the principle of capacitive sensing. Capacitive sensing works by sensing the proximity of an object, like a user's finger, that is conductive or has a different dielectric constant compared to the surrounding air.
When two conductive plates are placed in close proximity to one another with an isulator (or dielectric) between them, a capacitor is created when charge is applied. When the capacitance of the plates changes as a result of a finger coming into close proximity or contact with it, capacitive sense solutions detect the change in the capacitance load, its location, its direction if any, its duration, and so on. They then synthesize the gathered information into digital signals and pass the information to an associated processor for corresponding action(s). There are different techniques for sensing the capacitive change and many parameters that affect the system's ability to do so. The sections below investigate these areas.
To most accurately qualify a user's intentional touch on a control interface, a capacitive touch design must be sensitive enough to accurately detect the presence of the user's finger while also being immune to noise sources. This provides a responsive system and prevents false positives, and is generally specified as the signal to noise ratio or SNR.
SNR indicates the strength of the received signal produced by user's finger versus the amount of noise it is receiving. A higher SNR is advantageous because it means the capacitive touch system is able to better differentiate a user's touch versus the surrounding noise sources.
SNR can also incorporate the noise immunity of the overall system. A consistently low SNR relative to the specification shows that the system is likely susceptible to noise. This can be due to either the overall system design including ground traces, batteries, digital routes, etc., or the design of the capacitive touch system itself.
A lower SNR specification greatly indicates higher chances of misinterpreting noise for a user touch or human proximity, leading to potential system issues from fasle positive detections. These false detections lead to system actions that consume battery life and can result in a poor user experience.
In summary, solutions with higher SNR specifications are generally easier to design with because they allow for a greater margin of error than systems with lower SNR specifications.
Power consumption is an increasingly important buying consideration for consumers, even for devices that are plugged into the electrical outlets like white goods and home automation systems. Today's buyers are more aware of power efficiency ratings like "Energy Star," and are making buying decisions based on power efficiency claims. That said, in a wall-powered system like a microwave oven or washer/dryer set, the power consumption of the capacitive touch interface can be a very low priority since it will likely make very little difference to the overall system power consumption. However, in battery-powered applications like remote controls, saving a few milliamps can directly affect the user experience. A power-hungry device can lead to a very short battery life, and users do not want to change batteries, ever.
Generally, capacitive touch solutions spend a majority of their time in a low-power polling mode, constantly going through a sleep-wake-sense-sleep cycle to detect user proximity events. Some solutions use capacitive thresholds to reduce polling current, and remain in sleep mode until a certain capacitive charge threshold is met.
When an event does occur, capacitive sense solutions go into a high-power scan mode, sensing the changes in capacitance, converting that information to digital signals and passing it to an accompanying processor for post-processing and system actions. Upon completion of the user's action and associated capacitive sense scan, the device returns to its low power mode.
Capacitive touch solutions consume their peak current when performing the scan function. Smart designs are optimized to consume as little current as possible during scan by performing the scan as quickly as possible with innovative techniques.
Processing the signals from the capacitive touch solution in the accompanying MCU core can also consume large amounts of power. For this reason, integrated capacitive sensing peripherals on a multi-funtion MCU may consume less current if they operate in an autonomous mode. In other words, capacitive sense peripherals that can poll/scan/sleep/poll without waking the core will likely consume less power than capacictive sense peripherals that require the core's interaction.
Likewise, dedicated capacitive sensing solutions can operate very power efficiently by allowing the host MCU to remain in deep sleep mode while it monitors for user interactions. As a dedicated capacitive sense device, it can be optimized for very low power.
Some devices allow for programmable thresholds and current consumption during scan mode through programmable drive strength. This programmability facilitates fine-tuning a design to meet users' expectations while optimizing for the lowest possible current consumption. The key is to prototype the design and perform user testing to be certain that the settings are truly optimized for a broad set of users and their interaction styles versus current consumption.
Response time is the time it takes for the capacitive sense system to qualify and process a touch event - from the user's initial interaction with the control screen to the completion and/or confirmation of the user's desired action.
This can be a complex series of events that occurs between the capacitive sense system and the overall product. Careful system design can minimize the user's perceived response time from their input to system confirmation. Some tactics include lighting confirmation indicator on the touch screen or changing the color of the LED to indicate action receipt.
One example is a touch-control microwave where a user first touches a capacitive sense enabled "ON" button. The capacitive sense system needs to wake up, confirm the touch of the button, pass the command to the MCU core, which then processes it and perhaps lights up an LCD screen or "READY" indicator to confirm the oven is ready for the next inputs. This allows the user to quickly see confirmation of their action.
Another example could be a capacitive sense button to start a pool pump. In this case, the user touches an LCD screen to activate the pump, and the system passes the command to the engine while also passing a command to light an LED confirmation. In this way the user sees the confirmation quickly and does not need to wait for the pool pump to come on.
As the first contact point in the user's input to the overall product, the touch controller's response time needs to be very fast and very accurate. This allows the input to be passed to the processor quickly, subsequent actions to be taken, and user confirmation provided.
Silicon Labs Capacitive Sense Solutions
Designers adding capacitive sense functionality to their products need to consider whether to use a dedicated capacitive sense device or a general purpose MCU with integrated capacitive sense functionality. Each solution provides benefits that scale in importance depending on the target product, its price point, the product size, and the urgency of time to market.
Silicon Labs' "Charge timing" Capacitive Sense Technology
There are various types of capacitive sense technology. Silicon Labs pioneered an innovative capacitive sense technology called "charged timing" in the early 2000s. Charge timing uses an external capacitive plate that is charged and discharged periodically and compared to a reference capacitance value. Charge timing is very power efficient and generates a very high SNR.
With its low power and high SNR, charge timing is able to detect very small changes in capacitance while consuming very little power. Silicon Labs' charge timing technology is incorporated in its fixed-function, dedicated capacitive sense products and its multi-function MCUs with integrated capacitive sense peripherals.
TouchXpress - Fixed-Function Capacitive Sense Family
The TouchXpress family of fixed capacitive sense devices make adding capacitive sense to an existing or new product fast and easy. The family offers low-power, standalone capacitive sense solutions that easily hook up to capacitive plates and translate user interactions to GPIO outputs or I2C outputs. The family is designed for very low power autonomous operation, allowing higher power host MCUs to remain in deep sleep mode until users interact with the capacitive sense touch interface.
TouchXpress devices are a good fit for designs that need to add capacitive touch but must leave the existing MCU and code untouched. They are small, autonomous, cost effective, and very low power. TouchXpress devices are preloaded with production-quality capacitive sense firmware. The firmware incorporates decades of experience in capacitive sense technology and is thoroughly tested and guaranteed in each device across temperature and voltage. This is important because touch control firmware is generally easy for engineers to conceptualize and prototype on evaluation kits, but can be more temperamental to get into production.
However, the preloaded firmware is also customizable along several key parameters through the Xpress Configurator included in Simplicity Studio. Xpress Configurator allows designers to customize activation thresholds, gain, active mode and sleep mode scan periods, operating frequency, and drive strength. These customizable parameters facilitate design optimization, but do not affect the firmware's productions quality readiness.
Example Xpress Configurator Interface for TouchXpress Customization
The family also uses Simplicity Studio's Capacitive Sense Profiler tool to visualize the capacitive device's performance and output information such as detection time, touch thresholds, charge levels, etc.
Example of Capacitive Sense Profiler Interface
These Simplicity Studio tools allow designers to easily configure their systems to meet their users' needs and use cases. Each tool also provides helpful guidance on each capacitive sense setting and what it does for users that are still learning about capacitive sense solutions.
TouchXpress devices are a good fit for adding capacitive touch to any new or existing product. They are standalone, small, and cost effective, and allow capacitive touch to run autonomously to the host MCU, saving power and speeding time to market.
Sleepy Bee - EFM8 Microcontroller with Integrated Capacitive Sense
Some designs may need the capacitive sense to be integrated within the host MCU's functionality and footprint. This may save both cost and size, but may also bring complexity with code development as the capacitive sense functionality will share resources with other MCU requirements.
The low power Silicon Labs Sleepy Bee EFM8SB1 Microcontroller (MCU) family provides integrated capacitive sense functionality as a low-power, autonomous peripheral. In other words, the capacitive sense functionality runs without interrupting or waking the MCU core or other peripherals.
Sleepy Bee MCUs are optimized for low power designs, and targeted to battery powered systems. They use the same "charge timing" technology used in the TouchXpress family to offer up to 14 capacitive sense channels, in addition to a comprehensive set of targeted functionality for portable devices.
EFM8SB Highlighted Features
- Pipelined CIP-51 Core
- Fully compatible with standard 8051 instruction set
- 70% of instructions execute in 1-2 clock cycles
- 25 MHz maiximum operating frequency
- Up to 64 kB flash memory, in-system re-programmable from firmware
- Up to 4352 bytes RAM (including 256 bytes standard 8051 RAM and 4096 bytes on-chip XRAM)
- Internal LDO regulator for CPU core voltage
- Power-on reset circuit and brownout detectors
- I/O: Up to 24 total multifunction I/O pins:
- Flexible peripheral crossbar for peripheral routing
- 5 mA source, 12.5 mA sink allows direct drive of LEDs
- Clock Sources:
- Internal 20 MHz low power oscillator with ± 10% accuracy
- Internal 24.5 MHz precision oscillator with ± 2% accuracy
- External RTC 32 kHz crystal
- External crystal, RC, C, and CMOS clock options
- Pre-loaded UART bootloader
- Temperature range -40 to 85°C
- Timers/Counters and PWM:
- 32-bit Real Time Clock (RTC)
- 6-channel programmable counter array (PCA) supporting PWM, capture/compare, and frequency output modes with watchdog timer function
- 4 x 16-bit general-purpose timers
- Communications and Digital Peripherals:
- 2 x SPI Master/Slave
- SMBus/I²C Master/Slave
- External Memory Interface (EMIF)
- 16-bit/32-bit CRC unit, supporting automatic CRC of flash at 1024-byte boundaries
- Programmable current reference (IREF0)
- 10-Bit Analog-to-Digital Converter (ADC0)
- 2 x Low-current analog comparators
- On-Chip, Non-Intrusive Debugging:
- Full memory and register inspection
- Four hardware breakpoints, single-stepping
- Single power supply 1.8 to 3.6 V
- QFP32, QFN32, and QFN24 packages
Silicon Labs Sleepy Bee MCU Feature Set
Since the capacitive sensing functionality is integrated in the Sleepy Bee MCUs, the overall solution requires a smaller footprint and may be a lower cost solution than using two separate devices. Like the TouchXpress solutions, Sleepy Bee MCUs provide capacitive sense example firmware code, although it must be more customized than the TouchXpress code to accomodate other MCU functionality. Sleepy Bee MCUs are highly configurable using Simplicity Studio's Configurator, Capacitive Sense Profiler, and Power Profiler tools.
Designers adding capacitive sense to their products need to consider whether to use a dedicated capacitive sense device or a general purpose MCU with integrated capacitive sense functionality. Each solution provides benefits that scale in importance depending on the target product, its price points, the product size, and the urgency of time to market.
The table below provides a brief overview of some of the important considerations and how a dedicated device or general purpose MCU may meet their needs. Silicon Labs' capacitive sense devices offer both a dedicated solution with TouchXpress and a low power integrated solution in the Sleepy Bee MCUs for battery powered devices.
|EFM8SB MCUs with Integrated Capacitive Sense Functionality||TouchXpress Fixed Function Capacitive Sense Family|
|Reduced cost from integrated cpacitive sense functionality||Dedicated, autonomouse device may save power|
|More robust functionality may make design more elegant, but take longer to develop||Ease of use, production firmware and tools make adding capacitive sense fast|
|Reduced board space from a single IC||Dedicated IC makes adding capacitive sense an easy task for product differentiation|