Capacitive touch sensing MCU works and basic characteristics (1)

Now the electronic products, touch-sensitive technology increasingly greater attention and applications, and new technology and IC market.

At the same time, the high sensitivity of capacitive touch technology has developed rapidly, its main application in capacitive touchscreen and capacitance touch screen, but since the capacitor will be affected by temperature, humidity or grounding situation varies, so the poor stability, thus requiring robust performance IC, so as to better ensure a stable right touch-sensitive.

In response to market demand, from the United States in high-performance analog and mixed signal IC manufacturers Silicon Laboratories of innovation (hereinafter referred to as: Silicon Labs) company special launched C8051F7XX and C8051F8XX series MCU (microcontroller), specifically for capacitance touch sensors, the anti-noise performance and operation speed of very prominent on performance.

First, Silicon Labs company of capacitance touch series MCU

Current Silicon Labs company launched C8051F7xx and C8051F8xx, capacitance touch series MCU for high signal to noise ratio high speed features in the industry is particularly excellent performance.

At the same time, flexible i/o configuration, to design even more convenient. In addition, because the series MCU internal integrates a especial Capacitance to digital converter (CDC), so to be able to carry out high-precision Capacitance-to-digital conversion for capacitance touch functionality.

CDC's specific working principle:

As shown in Figure 1, IREF is an internal reference to the current source, CREF is internal integrated charging capacitor, ISENSOR belong to the internal integration of controlled current source, CSENSOR for external charging capacitor capacitance sensor, as the human touch of change, caused by CSENSOR through an internal adjustment to the CSENSOR ISENSOR moment of the charge, at a voltage on CSENSOR VSENSOR, then relative internal reference voltage after a zoom-mode differential amplifiers; similarly IC internal IREF after charging the CREF also produces a reference voltage VREF is relatively the same, and finally through the difference amplifier will 2 enlarged signal through the SAR (successive approximation ADC)-ADC sampling work out ISENSOR value.

Figure 1

Silicon Labs-SAR ADC sampling to select 12-16-bit resolution, as shown in Figure 2, uses 16 bits of resolution for bitwise comparison sample: first of all, from determining the MSB of 16 bits (IREF = 0x8000) began, the MSB value depends on the capacitor charging rate, equivalent to the current size, through current IREF/2, compare VSENSOR and VREF:

VSENSOR > VREF is the MSB = 0; VSENSOR < VREF is the MSB = 1;

Subsequently, the SAR control logic to move to the next, and set the bit is set to high level, for the next comparison:

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

This process continues until the least significant bit (LSB).

The operation is finished, it completed the conversion, will work out of the 16-bit conversion results stored in registers.

Figure 2

Use this capacitance acquisition conversion feature, available in capacitive touch screen or touch.

For example, the application of the capacitive touch screen (shown in Figure 3). General self-contained capacitance touch screen includes a layer of surface glass layer, the middle two rows of ITO layer (layers between no rows and shorted) and GND to the back. Each row and column collection with the MCU is directly connected to the input channel, when the finger touch capacitive screen surface glass layer, causing a row or column of ITO block to ground capacitor (fig. 4) values larger, thus through capacitor sampling as well as specific algorithms determine the capacitance value occurs necessarily change point (touch point) location (X, Y), the last position of the touch point to primary processors on the system operation functions.

Current Silicon Labs of C8051F7XX touch screen features mainly single points of touch, but through the software algorithm can achieve two points of gesture recognition, such as scaling, rotation, etc., while also identifying the drops of water and wet fingers touch normal dash features.

And touch the same sampling theory of capacitor, but each acquisition input channels connected to a touch screen, the MCU can determine if a key is touch and then make the appropriate function of the algorithm is relatively simple.

Figure 3

Figure 4

[1

] [2]