Hardware Sensor Configuration

Sensor Configuration Overview

The sensor configuration configures the Capacitive Voltage Divider (CVD) waveform, which is used to acquire the relative capacitance value of the sensor, and also the number of oversampling for each sensor. The differential CVD waveform is shown below:


There are two ADC samples being acquired. Each sample needs three phases: sensor pre-charge, sensor acquisition, and analog to digital conversion. For more information about the CVD acquisition method, see Application Note AN1478 mTouch™ Sensing Solution Acquisition Methods Capacitive Voltage Divider.

Common Sensor Configuration

There is a common configuration section to setup all the sensors, as shown below:


Individual Sensor Configuration

Each sensor has its own configuration section to set up the sensor name, Pre-charge Time, Acquisition Time, and Oversampling value. The common configuration is assigned to each sensor by default. If the sensor has different physical conditions (parasitic capacitance, size of the sensor, etc..), you can enable the individual configuration.


Sensor Waveform Configuration

Pre-charge Time: This value determines the amount of time provided to charge the external sensor and the internal Sample&Hold capacitor of the ADC. The unit is instruction cycle. If the value is too small, the sensor will not be as sensitive as they could be, or even has inverted behavior when touching the sensor.

Acquisition Time: This value determines the amount of time provided for the external sensor and internal hold capacitor to charge/average their voltages. The unit is instruction cycle. This value is related to the RC time constant of the sensor and its series resistor. If the value is too small so that the sensor charge is not fully settled before conversion, the sensor will not be as sensitive as it could be. If the value is too large, the noise immunity of the system will not be as robust as it could be, because this time is also the time window for acquiring noise.

Additional Sample Capacitance: The value determines how much additional internal capacitance will be added to the Sample&Hold capacitor. This is used to improve the match between internal and external capacitance for a better sensing performance. The general rule of thumb is to settle the voltage after acquisition time close to half of the VDD as much as possible. This option is only available on parts that have hardware CVD module.

Auto-Calibration: To ease the tuning process for the CVD waveform, the library supports auto-calibration for acquisition time and additional internal capacitance. The auto-calibration will perform for each sensor after power-up, timeout and negative capacitance detected events. This option is only available on parts that have hardware CVD module.

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