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Excerpt from datasheet:
uint32 CapSense_GetSensorCp(uint32 sensor)
This API returns the Cp (parasitic capacitance) of sensor in pF (pico farads).
The supported range is 5-100 pF. The precision of API is +/-10%.
Note This API can be used to measure a capacitance of the external capacitors. In this case the parasitic capacitance should be subtracted from the total result.
uint32 sensor: Sensor number. The Capsense.h file contains defines for the sensor numbers. See the Sensor Constants section for details.
uint32: This API returns Sensor parasitic capacitance (Cp) of the sensor indicated as argument. The unit of sensor Cp value is pico-farads.
Sensor scan should be complete before using this API.
What more do you need? An uint32 is returned.