3.1 Touch Measurement Operation

Principle of Touch Measurement

The touch sensor electrode forms a parasitic capacitance (Cx) between the sensor pad and the ground. Cx increases when the sensor electrode is touched.

The device performs touch measurements by using the charge transfer principle. Touch measurement is performed by charging the internal compensation capacitor (CC) and sharing the charge accumulated in CC with Cx. A 10-bit Analog-to-Digital Converter (ADC) monitors the voltage at the mid-point between CC and Cx. Any change in Cx changes the mid-point voltage (measured by ADC). This measured data is called “Signal”.

The touch measurement relies on charge transfer. CC or Cx must be fully charged before charge sharing starts to avoid inconsistent results and to increase sensitivity and noise suppression. The charge time depends on the RC time constant of Cx and the series resistor. Use the Charge Share Delay, and Measurement Clock frequency parameters to increase charge time as required. The respective sections provide more information.

The charging and discharging involve toggling the touch pins. The series of pulses is called bursting pulses. The frequency of these pulses is the bursting frequency. The configuration of Measurement Clock frequency, Charge Share Delay and Frequency Hop will impact the burst frequency.

Calibration

The sensor capacitance without touch can vary based on environmental conditions like temperature or humidity. The system must calibrate at power-up.

The calibration process involves two steps.

The CC is adjusted to match the Cx. When CC is identical to Cx, the mid-point voltage will be VCC/2. Thus, the resulting signal value is usually a multiple of 512, depending on digital gain settings.
If the CC and Cx aren’t perfectly equal, the resulting signal value will not be 512. Thus, the system cannot always rely on 512 values to detect touch. The signal value measured at power-up (after matching CC and Cx) is used for subsequent measurement. This value is called “Reference”.

Touch Detection

After calibration, touch measurement is performed periodically, defined via the Measurement Period parameter. After each measurement, the Delta (Delta = Signal – Reference) value is computed. If the Delta is higher than the threshold, Detect integration is performed to confirm a touch. Once the sensor goes in to detect and if the Delta drops below the threshold (actually, threshold – hysteresis), the detect integration performs to confirm the touch removal. Refer to sections Threshold and Hysteresis for more information.

Drift

Touch buttons are usually not touched over an extended period, while the touch event happens quickly. For example, a light switch is touched a few times a day for a very short time while being able to operate around the clock.

During round-the-clock system operation, changes in environmental conditions, such as Temperature and/or Humidity, are likely to happen. These changes impact the touch sensor electrode's capacitance.

Complex applications may have buttons that are physically closer to each other. Due to proximity, activation (touch) of one button might impact on the signal of adjacent buttons. In such scenarios, to avoid unintended environmental drift, drift is stopped on all the buttons when any of the buttons are activated.

Refer to the Drift Operation section for more information.

Touch Measurement Flow