18.1.2 Step 2: Charge Transfer Test
As an effect of adding a series resistor to form a low pass filter, the time constant for charging the sensors is increased. It is essential to ensure that the sensor capacitance is fully charged and discharged during each measurement sampling.
Insufficient charging can be observed as a reduced touch delta or compensation circuit calibration.
However, this problem may not be apparent in the touch sensor operation; the application may behave well even in the presence of low-level noise, but show much worse performance during noise tests with the addition of the resistor compared to a configuration which excludes the resistor.
Charge Transfer Calibration
The QTouch® Modular Library provides functionality to automatically adjust timing parameters to ensure full charge transfer.
Calibration may be configured to tune one of three parameters, depending on the target device and measurement technology.
CAL_AUTO_TUNE_RSEL- Clock prescaler and CSD are maintained at the configured
setting, while the internal series resistor is adjusted to the maximum
value, which allows adequate charging for each sensor node.
- Only available with PTC Mutual capacitance acquisition
CAL_AUTO_TUNE_PRSC- Series resistor and CSD are maintained at the configured
setting, while the prescaler is adjusted to the minimum value, which allows
adequate charging for each sensor node.
- Incrementing doubles the acquisition time, decrementing halves the acquisition time
CAL_AUTO_TUNE_CSD- Both Prescaler and Resistor are maintained at the configured
setting, and Charge Share Delay is adjusted to the minimum value, which
allows adequate charging for each sensor node.
- Incrementing CSD adds one cycle to the charge transfer phase of the acquisition sequence