Capacitive Sensing and Terminal CPU Solutions in Mobile Phones (1) 4

Period measurement utilizes the oscillator frequency as the gating 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 for measuring capacitance. Both methods can be easily implemented using a traditional 555 timer or a more complex microcontroller. Since determining the presence of a finger requires intelligent software algorithms, the microcontroller-based solution appears more sophisticated.

PSoC Mixed Signal Array

The CY8C21x34 and CY8C24794 series from Cypress's PSoC mixed-signal array employ CapSense technology, featuring a set of unique hardware capabilities. These capabilities enable cost-effective, straightforward capacitive sensing designs through a relaxation oscillator topology, as illustrated in Figure 4. The suite of features includes: a high-precision timer and counter, a comparator with automatic discharge switch connection, a programmable current source, and a versatile analog multiplexer that allows each pin to function as a potential capacitive sensor. Like all PSoC devices, peripherals required for specific applications are configured via internal registers. PSoCDesigner is a software tool designed to assist with PSoC application development, incorporating dedicated user modules and wizards to configure internal registers for capacitive sensing designs. This powerful combination provides engineers with the tools needed to implement simple capacitive digital switches (or even more complex capacitive linear sliders). Beyond internal register configuration, PSoCDesigner also includes essential software routines to determine whether a finger is touching the digital switch or to pinpoint the exact position of a finger on a linear slider. Such intelligence is indispensable for achieving a complete capacitive sensing application.  diode