Traditional home appliances usually use physical buttons to operate, but with the continuous advancement of technology, more and more home appliances are beginning to use touch screens and touch buttons to provide more intelligent and convenient operations.
This article introduces the touch detection principle of Fudan Micro FM33FR026. The
TSI
module uses self-capacitance method to detect touch behavior. The principle of self-capacitance detection is shown in the figure below. When the sensor PAD is not touched, the electric field of the sensor PAD and the wiring can only be coupled to the grid pavement, forming the static capacitance CS of the sensor. In the case of a finger touch, a capacitance CF to ground is formed between the sensor P AD and the finger through the covering layer, which makes the capacitance value of the sensor P AD increase. Therefore, the T SI module can detect touch behavior by detecting changes in the capacitance value of the sensor.
The TSI
module uses the Sigma Delta method internally to detect changes in the sensor capacitance value. The module first applies a switching signal with frequency fSW to the internal switch through the sampling clock, driving the sensor equivalent capacitance CP to charge and discharge (it should be noted that the fSW switching signal we give must at least ensure that it can be fully discharged). The power supply for charging and discharging comes from the energy storage capacitor CREF mounted on the TSCAP pin. At the same time, the voltage comparison circuit performs closed-loop control on the I DAC by sampling the voltage on the energy storage capacitor, so that the voltage on the energy storage capacitor is always stabilized at a fixed reference voltage. A simple diagram is as follows:
The equivalent resistance Req is calculated as follows:
The Sigma-Delta method reflects the amount of equivalent Req extraction current by accumulating the proportion of the modulation IDAC turn-on time to the entire cycle within a channel scan cycle. The TSI module uses a counter to quantify this ratio, and the quantization granularity can be changed through the resolution N configuration. Finally, the raw count value RawCount we obtain from the TSI module can be calculated as follows:
1. In single IDAC mode
2. In dual IDAC mode, the compensation IDAC is used to reduce the invalid count value introduced in RawCount by the static capacitance of the sensor line itself, and expand the effective range of RawCount, so that at the same resolution, other parameters can be adjusted to obtain greater accuracy:
It can be seen that the RawCount value has a linear function relationship with the total capacitance value of the sensor line, so we can directly use it to measure the capacitance value of the sensor.
Click to read the original text and learn more exciting content~