The acquisition level of the stack abstracts the hardware and acquires samples for the capacitive touch sensing.To perform the acquisition the MTouchAcquisition(…) function should be called periodically in the application. Depending on the hardware modules used on a PIC Microcontroller, the library supports two acquisition methods: capacitive voltage divider (CVD) and charging of the sensors using constant current source (CTMU).
- CVD: PIC Microcontroller's ADC holding capacitor (Chold) is used for the measurements. Initially the capacitive sensor (Csensor) is disconnected from Chold. Chold should be charged to Vdd and Csensor should be discharged. Then both capacitors are connected together to divide a charge between them. Capacitance of Chold is constant so the result voltage will depend on capacitance of Csensor. When sensor is touched the capacitance is increased and voltage is decreased. When the sensor is released the capacitance is decreased and voltage is increased. The minimum number of sensors required for this acquisition method is 2.
- CTMU: If the capacitive sensor will be charged by a constant current source during a constant time then the voltage on the sensor after the charge will depend on the capacitance. When sensor is touched the capacitance is increased and voltage is decreased. When the sensor is released the capacitance is decreased and voltage is increased.