With the increasing update and intelligent development of modern electronic products, human-computer interaction interface (HMI) has gained more and more attention and application, enriching people's experience, and as an important part, touch sensing technology is also fast. development of.

At present, touch technology is mainly divided into resistive touch and capacitive touch. As a new technology developed rapidly in recent years, capacitive touch sensing technology has no mechanical loss, long life, high sensitivity, space saving and touch. At the same time, semiconductor manufacturers are constantly introducing ICs of corresponding technologies to simplify the development of hardware designers.

Freescale Semiconductor's KineTIs family of MCU architectures incorporates a high-performance capacitive touch-sensing interface TSI module that enhances the stability and robustness of capacitive touch sensing while greatly simplifying the designer's development process.

Capacitive touch sensing principle

At present, there are two main types of capacitive touch sensing technology based on IC design:

One is to convert the change of the capacitance value into a change of the voltage, and then calculate the capacitance through A/D sampling through an internal special capacitance analog-to-digital converter;

The other is to convert the change in capacitance value into a change in the count value of the internal counter, generate a triangular wave charge and discharge voltage signal on the external electrode, and measure the count of the triangular wave voltage signal to reflect the change in capacitance of the external electrode.

The capacitive touch series MCU from SiliconLabs uses the former method.

The KineTIs K60's integrated TSI module uses the latter method.

The TSI module charges and discharges the external electrodes through an internal constant current source to form a triangular wave voltage signal. The period of the triangular wave voltage signal changes with the change of the external capacitance, and when the finger is virtually close to the electrode, the capacitance capacity is increased, and the period of the triangular wave voltage signal is lengthened. At the same time, the TSI module also has an internal oscillator with a fixed capacity capacitor, which counts the period of the triangular wave voltage signal generated by the external electrode with the reference clock tempo generated by it, and the change of the external electrode capacitance causes the triangular wave voltage signal period. The change in the measurement causes a change in the measured count value, and the capacitance change can be calculated by internally reading the corresponding counter value. According to the internal operating mechanism of the TSI, when the capacitance value exceeds the set trigger threshold, the TSI trigger flag activates the corresponding interrupt request to achieve a response to the capacitive touch sensing event.

System hardware design

The design of the hardware circuit is simplified by the use of an MCU with a dedicated capacitive touch function, the TSI module. On the one hand, the development cost is reduced, on the other hand, the complexity of the hardware circuit is reduced, and the stability and robustness of the system are enhanced.

1. Capacitive touch interface design

The built-in TSI module interface greatly simplifies the hardware design by simply connecting the external electrodes through a current limiting resistor to the corresponding TSI module channel. The current limiting resistor is mainly used to prevent the MCU from being damaged due to excessive charge and discharge current between the electrode and the MCU. The size of the current limiting resistor depends on the actual situation.

2. Touch keyboard PCB layout design

The copper clad plate with insulating varnish is used as the electrode plate, and the triangular shape copper splicing is combined into a square touch keyboard, and the triangular copper is respectively led out to the corresponding interface of the TSI module.

System software design

The TSI module not only simplifies the design of the capacitive touch keyboard hardware, but also drives the capacitive touch function by simply configuring the relevant registers, writing the corresponding calibration program and interrupt service program in software design, which greatly simplifies the software design process. . The system software is designed to use the interrupt mode. The TSI module automatically performs periodic scans. Only when a touch event occurs, the interrupt is triggered to respond, reducing the CPU load.

1. TSI module initialization

The initialization of the TSI module is mainly based on the actual hardware design to operate its related registers. The related registers involved in the initialization phase include the general control and status register (TSI0_GENCS), the scan control register (TSI0_SCANS), and the channel enable register (TSI0_PEN). In addition, it also involves the K60MCU internal clock and pin configuration registers.

2. TSI module self-calibration

After the TSI module is initialized, to realize the detection of the capacitive sensing touch, it is necessary to calibrate the capacitance value of the TSI module, and the capacitance of the normal counter without finger touch, that is, the count value of the internal counter, and the custom dead zone. After the values ​​are added and subtracted, they are stored in the high and low portions of the threshold register respectively. This is used as the standard detection electrode capacitance change interval. When the capacitance change is in the dead zone interval, the cross-border interrupt will not be triggered. An out-of-bounds interrupt is automatically triggered when the capacity exceeds the range of the threshold register, including below the low portion of the threshold register or above the high portion of the threshold register.

3. TSI module interrupt service processing

The TSI module has multiple interrupt modes, including error interrupts, timeout interrupts, scan end interrupts, and out-of-bounds interrupts. In the K60 MCU internal interrupt mechanism, they share the 99th interrupt vector.

At the same time, Freescale provides a powerful touch sensing software library (TSS library) and development ecosystem support for free, which can be directly applied to Freescale KineTIs platform, which shortens the engineering development cycle and increases the system's stability.

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January 08, 2021