Charge transfer

The acquisition is based on the measurement of the sensor channel capacitance.

To ensure that the C_x capacitance is correctly charged, it is necessary to monitor the pin connected to the sensor. On the sensors and shield sides, it must observe a complete Charge/Discharge cycle.

In this example, to complete the charge transfer cycles, the following parameter must be modified as below:

• INCREASE:
  • htsc.Init.PulseGeneratorPrescaler
  • htsc.Init.CTPulseHighLength
  • htsc.Init.CTPulseLowLength

• DECREASE:
  • Sysclk

Table 5 Charge transfer documentation gives a list of documents containing information about the charge transfer.

Sensitivity is a key point in touch sensing applications. The sensitivity can be improved by:

• reducing the air gap,
• reducing the panel thickness,
• choosing the dielectric with higher ε_R ,
• a GND plane that is not too close to the shield and sensors,
• avoiding metallic paint near shield and sensors.

Table 6 Sensitivity documentation gives a list of documents containing information about the sensitivity.

Dielectric example

• It is recommended to use the same shape for all electrodes.
• The touchkeys can be customized by the drawing on the panel. TSL compensates capacitance differences.
• Acquisition time and processing parameters can be optimized when electrodes have similar capacitance.

Sensor size example

• Key sensors are used in common application

• You can get deeper key information in the following documents:

Table 8 Key documentation gives a list of documents containing information about the key.

A linear is a set of contiguous capacitive electrodes. Figure 9 Interlaced linear touch sensor with three channels / four electrodes (half-ended electrodes design) shows a slider used on a discovery board.

Table 9 Linear touch sensor documentation gives a list of documents containing information about the linear touch sensor.

A rotary is a set of contiguous capacitive electrodes.

Table 10 Rotary sensor documentation gives a list of documents containing information about the rotary sensor.

Active shield or driven shield. (this name is used in some application notes) drives the shield plane with the same signal as the electrode.

There are several advantages using Active Shield instead of a grounded shield:

• The parasitic capacitance between the electrode and the shield no longer needs to be charged.
• Protect the touch electrodes from a noise source.
• Increase system stability and performance when a moving metal part is close to the electrode.

Table 11 Active shield documentation gives a list of documents containing information about the active shield.