There are many different techniques to measure the change in self-capacitance which is a conductive plate’s capacitance to its surroundings. Resistor-capacitor charge timing measures how long it takes to charge or discharge a capacitor. In this case a capacitor is discharged, then a voltage is applied to the RC network and the time it takes to cross a threshold is measured. Another method, charge transfer, involves charging a sensor capacitor and then transferring that charge into an integrating capacitor over several cycles. The voltage change on the capacitors gets exponentially smaller. The integrating capacitor voltage is compared to a reference voltage or measured with an ADC and the time associated with it is determined. The relaxation oscillator method builds an oscillator which uses the sensor capacitor as a timing element. In order to build the oscillator it is necessary to charge the sensor capacitor to a specified upper threshold voltage. Once that value is reached, the sensor capacitor is discharged to a specified lower threshold voltage. In the relaxation oscillator method the oscillation period is inversely proportional to the sensor capacitor value. As capacitance increases the oscillator frequency decreases. As shown in the next slide, the C8051F70x/71x/800/99x family uses a successive approximation technique to compare the sensor capacitor ramp with a known time base.