CapSense Properties – CSDR (Capacitive Sensing – Sigma-Delta Modulator) Algorithm (Support CY8C24x94 Family)

Functional Description

This CapSense CSDR Property driver configures the capacitive sensing algorithm properties common to all CapSense – CSDR drivers in a project. This driver implements the Capacitive Sensing – Sigma-Delta Modulator (CSD) sensing algorithm.

Hardware Interface: All CSDR input drivers in your design will share an external modulation capacitor (ModulatorCapPin) and a modulator feedback resistor FdbkResistorPin). The capacitor can be connected between Vss ground and the P0[5] or P0[7] port pin. The feedback resistor can be connected between the capacitor pin and the P1[1], P1[5], P3[1], P3[5], P5[1] or P5[5] port pin. The pins are selected during pin assignment. Please do not use pins selected for modulator component connections for any other purposes.

The Feedback resistor value depends on the total sensor capacitance. The resistor value should be selected as follows:

1. Monitor the raw counts for different sensor touches.

2. Select a resistance value that provides maximum reading with a finger present that is about 70% of the maximum possible reading for the selected scanning resolution (for example 10 bit resolution has a maximum possible reading of 1023). The raw counts increase as the resistor value increases.

Typical values are 500 Ω – 10 k Ω depending on sensor capacitance.

The recommended value for the Modulation capacitor is 4.7 – 47 nF. The optimal capacitance can be selected by experiment to get maximum SNR. A value of 5.6 – 10 nF gives good results in the most cases. You can experiment with several capacitor values to get the best SNR after selecting the feedback resistor value. A ceramic capacitor should be used. The temperature capacitance coefficient is not important.

Software Interface: There is no software interface for this driver.

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Driver Properties

User Configurable Properties:

· NoiseThreshold: Used to determine if the baseline update algorithm is run. When the raw count value is above this level, the baseline value is not updated.

For slider sensors, this value sets the raw count level below which a sensor element in not included in the calculation of the centroid. The Noise Threshold value can be from 5 to 255.

The Noise Threshold value for a project must be set based on the amount of variation observed in the Raw Count values of the CapSense drivers when no finger is present. All CSDR based capacitive sense drivers in a project use this threshold value.

· BaselineUpdateThreshold: Determines when the Baseline count value will be updated. When the raw count value is above the current baseline and the raw count/baseline count difference is below the noise threshold, a running sum of the difference values is maintained. When this sum exceeds the BaselineUpdateThreshold value, the baseline value is incremented and the running sum is cleared.

· NegativeNoiseThreshold: The NegativeNoiseThreshold parameter adds a negative difference count threshold. If the current raw count is below the baseline and the difference between them is greater than this threshold, the baseline will not be updated. However, if the current raw count stays in the low state (difference greater than threshold) for the number of samples specified by the LowBaselineReset parameter, the baseline will be reset.

· LowBaselineReset: The NegativeNoiseThreshold parameter adds a negative difference count threshold. If the current raw count is below the baseline and the difference between them is greater than this threshold, the baseline will not be updated. However, if the current raw count stays in the low state (difference greater than threshold) for the number of samples specified by the LowBaselineReset parameter, the baseline will be reset.

· Sensors Autoreset: This parameter determines whether the baseline update algorithm operates continually, or only when the difference between the sensor raw count and the current baseline value is below the Noise Threshold. The baseline is updated constantly when this property is Enabled.

When the parameter is set to Disabled, the baseline is updated only when raw count and baseline difference is below the Noise Threshold property.

This setting limits the maximum time a sensor can be active (typical values are 5 - 10s), but prevents the sensors from being triggered and “stuck” in the on state by a sudden rise in the raw count value when no contact is made with the sensor (due to a large power supply fluctuation, high-energy RF noise source or rapid temperature change).

This property should be set to Enabled unless there is a requirement for sensors to be in the on state for extended periods.

· Hysteresis: Controls the de bounce (button on transition) or button hold time (off transition) of the sensors. The hysteresis value adds or subtracts from the finger threshold value defined for each sensor. If the sensor is off, the difference count must overcome the finger threshold plus hysteresis to be considered active. If the sensor is on, the difference count must go below the finger threshold minus hysteresis to transition to the off state.

The hysteresis value can be 0 to 255. This value must be lower than the FingerThreshold property value for any of the CSDR sensors in the project.

· Debounce: This parameter adds a debounce counter to the sensor active transition. In order for the sensor to transition from inactive to active the difference count value must stay above the finger threshold plus hysteresis for the number of samples specified. This parameter has effect on the buttons touch detection only.

Possible values are 1 to 255. A setting of 1 provides no debounce.

PRS Polynomial: This parameter sets the PRS polynomial. There are two selection options:

• Short - The short polynomial setting yields better SNR, but due to the shorter repeat period, the end device can be more susceptible to external noise sources.

• Long - The long polynomial setting yields worse SNR, but the device is more robust against noise signals.

Non-Configurable Properties:

This driver has no Non-Configurable properties

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Interface Schematic

Components in this schematic support the sensing for all CapSense – CSDR sensors in a project.

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