Working Principle and Basic Features of Capacitive Touch Sensing MCU

In today's electronic products, touch sensing technology is increasingly gaining more attention and application, and new technologies and ICs are constantly being introduced. At the same time, high-sensitivity capacitive touch technology is also developing rapidly, and is mainly used in capacitive touch screens and capacitive touch buttons. However, since capacitance will change with temperature, humidity or grounding conditions, its stability is poor, so the IC's anti-noise performance is required to be good in order to ensure stable and correct touch sensing.

In response to market demand, Silicon Laboratories (Silicon Labs), an innovative manufacturer of high-performance analog and mixed-signal ICs from the United States, has launched the C8051F7XX and C8051F8XX series of MCUs (microcontrollers), which are specifically designed for capacitive touch sensing and have outstanding performance in noise resistance and computing speed.

1. Silicon Labs' capacitive touch MCU series

At present, the capacitive touch MCU series such as C8051F7xx and C8051F8xx launched by Silicon Labs have performed particularly well in the industry with their high signal-to-noise ratio and high speed. At the same time, the flexible I/O configuration brings more convenience to the design. In addition, since the series of MCUs integrate a special capacitance-to-digital converter (CDC), it can perform high-precision capacitance-to-digital conversion to realize the capacitive touch function.

How CDC works:

As shown in Figure 1, IREF is an internal reference current source, CREF is an internal integrated charging capacitor, ISENSOR is an internal integrated controlled current source, and CSENSOR is the charging capacitor of the external capacitive sensor. The touch of the human body causes the change of CSENSOR, and the internally adjusted ISENSOR is used to instantly charge CSENSOR, generating a voltage VSENSOR on CSENSOR, which is then amplified by a common-mode differential amplifier relative to the internal reference voltage. Similarly, after IREF inside the IC charges CREF, it also generates a reference voltage and is differentially amplified relative to the same VREF. Finally, the two amplified signals are sampled by a SAR (successive approximation analog-to-digital converter) ADC to calculate the value of ISENSOR.

Figure 1

Silicon Labs SAR ADC sampling can select 12-16 bit resolution, as shown in Figure 2. 16-bit resolution is used for bit-by-bit comparison sampling: First, start by determining the highest bit 16 (IREF=0x8000). The value of the highest bit depends on the charging rate of the capacitor, which is equivalent to the current. Take the current IREF/2 and compare VSENSOR and VREF:

If VSENSOR > VREF, the highest bit = 0;
if VSENSOR < VREF, the highest bit = 1;

The SAR control logic then moves to the next bit and sets that bit high for the next comparison:

If the 16th bit is 1, then take the next IREF=0xC000;
if the 16th bit is 0, then take the next IREF=0x4000.

This process continues until the least significant bit (LSB). After the above operations are completed, the conversion is completed and the calculated 16-bit conversion result is stored in the register.

Figure 2

This capacitance acquisition conversion function can be used on capacitive touch screens or touch buttons. For example, the application of capacitive touch screens (as shown in Figure 3). Generally, self-capacitive capacitive touch screens mainly include a surface glass layer, two layers of ITO layers with crossed rows and columns in the middle (there is no short circuit between the row and column layers), and a GND bottom layer. Each row and column is directly connected to the acquisition input channel of the MCU. When a finger touches the surface glass layer of the capacitive screen, it will cause the capacitance of the ITO block of a row or column to the ground (as shown in Figure 4) to increase, thereby determining the position (X, Y) of the point (touch point) where the capacitance value changes to a certain extent through capacitance sampling and a specific algorithm, and finally uploading the position of the touch point to the main processor to realize the system operation function.

Currently, Silicon Labs' C8051F7XX touch screen mainly supports single-point touch, but through software algorithms, it can realize two-point gesture recognition, such as zooming, rotating, etc. It can also realize water drop recognition and normal stroke function of wet finger touch.

The capacitance sampling principle of touch buttons is the same, except that each acquisition input channel is connected to a touch button. The MCU can directly determine whether a button is touched and then implement the corresponding function. The algorithm processing is relatively simple.

Figure 3

Figure 4

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3. Advantages and features of Silicon Labs touch series MCU

1. High signal-to-noise ratio

The capacitive sensor module first releases the charge of the external capacitor and then calculates its charging speed to determine the changing capacitance value. Therefore, before each measurement, the remaining charge of the capacitor must be completely released to ensure more accurate measurement.

Whether the discharge of external capacitors is thorough directly affects the anti-noise performance. General MCUs discharge through a resistor connected to ground, while Silicon Labs' MCU performs two-stage capacitor reset discharge before each bit conversion: first, the first-stage discharge is performed by connecting a small-resistance resistor to ground, releasing most of the residual power of the capacitor, and then turning to the second-stage reset release, connected in series with a high-resistance resistor to ground, completely eliminating the noise energy that may be generated at the end of the first-stage reset release. The two-stage capacitor reset release can fully eliminate the influence of environmental noise, thereby greatly improving the conversion signal-to-noise ratio.

The traditional method of calculating the signal-to-noise ratio is to calculate the difference between the average capacitance value AvgA measured when the finger is touching and the capacitance value measured when idle, AvgI, and then the ratio of the peak value NoiseI of the capacitance caused by the noise when idle:

Currently, the industry can only achieve a signal-to-noise ratio of 80:1, while the signal-to-noise ratio of Silicon Labs' touch series MCU can reach 99.7:1 (as shown in Figure 5). The high signal-to-noise ratio ensures that misoperation is reduced to a greater extent, while the sensitivity is also greatly improved.

Figure 5

2. High speed

Silicon Labs MCU uses a 3-stage pipeline instruction structure. 70% of the instructions only need 1 or 2 system clock cycles to execute. The CPU speed can reach 25MIPS. The fastest conversion time for each channel is only 40us. If there are 27 channels, a scan only takes 1.08ms. The efficient conversion rate can improve the system's work efficiency and allow users to experience the effect of speed.

3. Flexible I/O configuration

The I/O ports of Silicon Labs' MCU can be configured arbitrarily through software according to the needs of designers. Unlike some other MCUs, some functional I/Os are fixed, which causes many staggered lines in LAYOUT and brings trouble to the design. In addition, C8051F700 can have up to 38 capacitive conversion input channels. The rich channel input brings more convenience to the design of capacitive touch applications and stronger compatibility.

4. Performance comparison of Silicon Labs capacitive touch MCU series with other competitors

The capacitive touch sensing working principle of C8051F7xx and C8051F8xx is the same, but the input channels of C8051F8xx are relatively less, with a maximum of 16 channels, so it is more suitable for touch button applications that require more flexible input channels, while the input channels of C8051F7xx can reach up to 38 channels, which is more widely used. It can be applied to both capacitive touch screens and capacitive touch buttons. In current consumer electronic products, it can be applied to mobile phones, washing machines, set-top boxes, office products, etc. At the same time, it can also be widely used in industrial fields such as touch panels, thermostats, security systems, and ATMs.

For the C8051F7xx and C8051F8xx series products and applications, Silicon Labs authorized distributor Shiqiang Telecom can provide corresponding DEMO boards and complete supporting software, information and technical support.