How Does CapSense Work?
All PSoC MCUs support CapSense capacitive-sensing. The figure below shows the self-capacitance of each electrode modeled as CSX and the mutual-capacitance between the electrodes modeled as CMXY. The CapSense circuitry internal to PSoC MCUs converts these capacitance values into equivalent digital counts (raw counts). These digital counts are then processed by the CPU to detect touches.
PSoC MCU, Sensors, and External Capacitors
CapSense also requires external capacitor CMOD for self-capacitive sensing and CINTA and CINTB capacitors for mutual-capacitive sensing. These external capacitors are connected between a dedicated GPIO and GND.
The capacitance of the sensor pad in the absence of touch is called the parasitic capacitance (Cp). Cp results from the electric field between the sensor (including the senor pad, traces, and vias) and other conductors in the system such as GND planes, traces, and any metal in the product’s chassis or enclosure. The GPIO and internal capacitances of PSoC also contribute to the Cp. However, these internal capacitances are typically very small compared to the sensor capacitance.
To find out more, checkout the PSoC 4 and PSoC 6 MCU CapSense Design Guide.