Fujitsu SK-8FX-TSC-32PMC Manuale d'uso - Pagina 23

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This capacitor is called 'Tuning Capacitor' (C14 on the SK-8FX-TSC-32PMC). Also here,

one step of Reference Delay register corresponds to 0.078pF when using the default

settings, so that attaching a 4.7pF capacitor to the PAREF pin will decrement all impedance

values by about 60 counts.

The maximum value for the reference tuning capacitor can be found in the device data

sheet.

The resolution of the above registers can be set using the R_SEL options. Please refer to

the register description in the documentation (on CD) for details.

5.2 Alpha

The Alpha parameter is used to control the sensitivity of every channel individually. In idle

state (no touch), the AIC will set the Reference Impedance value for every channel to be

(Impedance + Alpha). This means, the signal introduced by a finger in order to be

recognized as a touch has to increase the Impedance value by at least 'Alpha' counts. In

other words, increasing the Alpha for a channel makes it less sensitive, a smaller Alpha will

make the channel more sensitive.

The Alpha values can be set using the following commands:

sa channel value – Set a single channel's alpha to value

•

sax value – Set alpha of all channels to value

•

a-value – decrement all alpha by value

a+value – increment all alpha by value

•

Typical values for Alpha are in the range of 4...30, depending on electrode size, front cover

thickness, etc. For proximity applications, also smaller values of Alpha can be used, but care

must be taken that AIC is still operating correctly. Also, the Filter Period and Filter Threshold

should be used to filter out short pulses and increase stability.

5.3 Filter Period, Filter Threshold

The Filter Period and Filter Threshold registers can be used to apply a kind of low-pass filter

to the touch output. A touch will only be signalled, when at least 'Filter Threshold' samples of

'Filter Period' samples are active. This can be used to increase stability, ot to filter out short

touches. It should usually be kept active, e.g. with Period = 5 and Threshold = 4. The filter

function has to be enabled by setting the FILTER_EN bit in the Feature Select register

(0x13).

5.4 Impedance, Calibrated Impedance

In addition to reading out the digital APIS on/off information for every touch channel, the TSC

offers access to the sensor reference and input impedance values. This can be used e.g. for

slider, scroll wheel or touch pad applications.

The Impedance register contains the value of the measured input impedance, whereas the

Reference Impedance register contains the corresponding reference value for each channel.

The difference between both in idle state is equal to the corresponding Alpha value, which is

handled by the AIC by setting the Reference Impedance value accordingly.

This means that the Alpha parameter and also AIC operation (calibration) do not affect the

value in the Impedance register. The Impedance register always reflects the input

SK-8FX-TSC-32PMC User Guide

Chapter 5 Basic TSC Tuning Process

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