33.4.9.2 Precharge Control
The precharge stage is the period of time that brings the external channel and internal Sample-and-Hold capacitor to known voltage levels. Precharge is enabled by writing a nonzero value to the ADPRE register. This stage is initiated when an ADC conversion begins, either from setting the GO bit, a Special Event Trigger, or a conversion restart from the computation functionality. If the ADPRE register is cleared when an ADC conversion begins, this stage is skipped.
The Precharge Sample Capacitor Only (PCSC) bit can be used to disable the precharge stage to the external channel.
During the precharge time, CHOLD is disconnected from the outer portion of
the sample path that leads to the external capacitive sensor and is connected to either
VDD or VSS, depending on the value of the PPOL bit. At the same time, when PCSC is clear (PCSC = 0),
the port pin logic of the selected analog channel is overridden to drive a digital high
or low out, to precharge the outer portion of the ADC’s sample path, which includes the
external sensor. The output polarity of this override is determined by the PPOL bit such
that the external sensor cap is charged opposite that of the internal CHOLD cap. If PCSC is set (PCSC =
1), the outer portion of the ADC’s sample path is disconnected,
preventing the precharge from occurring on the external channel. The amount of time for
precharge is controlled by the ADPRE register.