Capacitive Sensing and Terminal CPU Solutions in Mobile Phones 3

The 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. Periodic measurement utilizes the oscillator frequency as the gate signal for the pulse width modulator (PWM) (Figure 3). A frequency higher than the relaxation oscillator frequency is used to time the timer. The faster the timer clock, the higher the resolution of measuring capacitance. Both methods can be easily implemented using traditional 555 timers or more complex microcontrollers. Due to the need for intelligent software algorithms to determine the presence or absence of fingers, solutions based on microcontrollers seem to be more sophisticated. The PSoC mixed signal array CY8C21x34 and CY8C24794. html "target=" -blank "title=" CY8C24794 ">CY8C24794 series products use CapSense technology and have a unique set of hardware features. It achieves a cost-effective and simple capacitive sensing design through a relaxation oscillator topology. Figure 4 shows this topology structure. This set of features includes: multiple precision timers and counters, comparator with automatic discharge switch connection, programmable current source, and a universal analog multiplexer that makes each pin a potential capacitive sensor. Like all PSoC, the peripherals required to implement specific applications are configured through internal registers.  diode