Fujitsu SK-8FX-TSC-32PMC Manuale d'uso - Pagina 23
Sfoglia online o scarica il pdf Manuale d'uso per Scheda madre Fujitsu SK-8FX-TSC-32PMC. Fujitsu SK-8FX-TSC-32PMC 28.
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
© Fujitsu Semiconductor Europe GmbH
SK-8FX-TSC-32PMC User Guide
Chapter 5 Basic TSC Tuning Process
- 23 -
FMEMCU-UG-90003-10