Principles and Applications behind Silicon Labs Capacitive Touch Sense Technology
Principles and Applications behind Silicon Labs Capacitive Touch Sense Technology
Author: Joe Chen, EDOM Technology
Touch sense interfaces have been shown a large base of adoption by the consumers’ electronic products recently to gradually replace the traditional mechanical switches. In order to satisfy the needs from the market requirements, Silicon Labs has announced a capacitive touch sense total solution with embedded MCU functions. The capacitive touch sense switch is a switch based on capacitors inside. Touching the capacitor with a conductive object (e.g. finger) changes the capacitance which can then be detected by circuitry built into the MCUs.
◆Silicon Labs currently offers one approach for detecting the change of capacitance caused by a touch
The basic principle of capacitive sense switch is based on a relaxation oscillator which keeps charging and discharging. When no touch is performed, the relaxation oscillator will maintain a fixed charging and discharging cycle. The frequency can be detected. When we use our finger or stylus to touch the switch, the dielectric of the capacitor will be increased, thus the charging discharging cycled will be increased accordingly and the frequency will be decreased. We can detect if touch is performed by measuring the change of the cycles.
A touch event can therefore be detected simply by measuring the change in frequency. There are two ways to measure this change:
1. Measure the Frequency
Count the number of relaxation oscillator cycles over a fixed period of time. If the number of measured cycles over the fixed period of time is lower than a pre-calibrated threshold, the switch is considered pressed
2. Measure the Period
Count the number of system clock cycles over a fixed number relaxation cycles. If the switch is pressed, the relaxation oscillator frequency decreases and so more system clock cycles are measured over those same numbers of cycles
The Silicon Labs’ newly-released C8051F9xx MCU family implements touch sensing via the use of on-chip comparator and timers. It can connect as many as 23 sense switches, and no external components are needed. The PCB traces/switches act as a capacitive element that is then measured by the internal touch sense circuitry for changes in capacitance
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◆ Implementing Touch Sense with Silicon Labs’ MCU
The Capacitive Touch Sense solution can be implemented on other Silicon Labs MCU families using only passive components. The only extra requirements compared to the C8051F93x-F92x solution are the (3 + N) resistors, where N is the number of switches and 3 extra port pins for the feedback. Silicon Labs MCU families other than the C8051F93x-92x family can interface up to 12 switches directly or more with an external analog multiplexer
Designing Touch Sensitive Switches
Since we are detecting a change in capacitance, it follows that we want to maximize the change as much as possible. There are three main factors that affect the switch capacitance and how much it changes:
1. Size, shape and placement of the switch pattern on the PCB
2. Type of material between the PCB trace and the user’s finger
3. The characteristics of the trace that connects the switch to the MCU
12 different switches were tested varying the sizes and shapes have been tested shown in the following figure. The purpose of the testing was to find out how the size and shape of the switch will have impact on the statuses in idle and touched. We can also discover which switch will have the largest idle capacitance. The larger the idle capacitance is, the less influence the capacitors on the PCB will have. From the results of our testing, it is easy to see that the bigger the switch and the more traces there are within a certain area, the more idle capacitance the switch has. In the figure, the ring shape switches have the lowest capacitance, when the switches are activated; larger capacitance changes will be detected.
◆How the shape and size of the switch can affect the idle and touched statuses
The type of material on top of the switch affects the idle capacitance and the active change in capacitance. We have performed tests on glass, Plexiglass, Mylar, ABS plastic and FR4. We have the conclusion that using the thinnest material possible can maximize the change in capacitance, and using materials with a higher dielectric, such as glass, can increase the absolute capacitance of the switch.
Summary
There are so many benefits of Silicon Labs’ capacitive sense switch solutions. First, very low MCU overhead is required. Only 443 bytes code space is needed for 1 switch and only 1 additional byte for each additional switch. Only 1 comparator and 1 timer are needed for the hardware resource. We can also use the efficient algorithm, which allows the MCU to go into low-power mode and wake up periodically to detect a switch event, and the CPU utilization is as low as less than 5%.
Furthermore, there is no external hardware overhead, it can directly connect the switch traces to the MCU port pins, no other external feedback resistors or capacitors are needed
In addition, the chip configuration is very simple. It is easy to perform with any material on the switch. The switch detection is insensitive to noise and supply voltage, it is insensitive to 50/60 Hz noise, and does not require precise source voltage (VDD).
For more detailed product information, please click on C8051F9xx MCU or email to sales@edom.com.tw
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