Capacitive Sensing and Terminal CPU Solutions in Mobile Phones (1) 3

At the beginning, the discharge switch is in an open circuit state, and the current source linearly charges the capacitive sensor. The voltage on the capacitive sensor rises diagonally until it exceeds the threshold of the comparator. The output of the comparator changes from low level to high level, causing the discharge switch to close. The capacitive sensor rapidly discharges to ground through this low impedance channel. This process will cause the output of the comparator to go from high to low, and then repeat the cycle. The output frequency (fout) depends on the charging current and the value of the capacitive sensor, and the relationship is as follows:

A typical design sets the oscillator frequency within the range of 20kHz to 200kHz, and then feeds this frequency into an intelligent measurement circuit. By measuring the change in frequency, the presence of fingers can be determined. There are two widely used methods for measuring the frequency of a relaxation oscillator: one is to measure the frequency, and the other is to measure the period. diode

Figure 2: Circuit and waveform for frequency measurement.

Frequency measurement refers to using a gate controlled timer to measure the frequency of a relaxation oscillator over a fixed period of time (Figure 2). Determine the accumulated count value in a fixed time by reading the timer. This method is suitable for low-frequency applications where the discharge time of capacitive sensors can be neglected compared to the oscillator period. diode