QST104 STMicroelectronics, QST104 Datasheet - Page 8

• Manufacturer Part Number: QST104
• Description: Capacitive touch sensor device 4 keys
• Manufacturer: STMicroelectronics

QST touch sensing technology
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Faulty and unused keys
Any sensing channel that does not have its sense capacitor (C
either faulty or unused. This channel takes no further part in operation unless a Master-
commanded recalibration operation shows it to have an in-range burst count again. Faulty,
unused or disabled keys are still bursted but not processed to avoid modifying the sensitivity
of active keys.
This is important for sensing channels that have an open or short circuit fault across C
Such channels would otherwise cause very long acquire bursts, and in consequence would
slow the operation of the entire QST device.
To optimize touch response time and device power consumption, if some keys are not used,
we recommend to try suppressing the ones which belong to the same burst. Bursts which
do not have any keys implemented will then not be processed.
Detection threshold levels
The key capacitance change induced by the presence of a finger is sensed by the variation
in the number of charge transfer pulses to load the capacitor. The difference in the pulse
count number is compared to a threshold in order to detect the key as pressed or not.
Two different thresholds, one for detection and one for the end of detection, create an
hysteresis in order to prevent erratic behavior.
The default threshold levels and hysteresis values are described
sensing characteristics on page
Detection integrator filter
Detect Integrator (DI) filter mechanism works together with spread spectrum operation to
dramatically reduce the effects of noise on key states. The DI mechanism requires a
specified number of measurements that qualify as detections (and these must occur in a
row) or the detection will not be reported.
In a similar manner, the end of a touch (loss of signal) also has to be confirmed over several
measurements. This process acts as a type of “debounce” mechanism against noise.
The default DI value for confirming start of touch and end of touch is described in
Section 6.5: Capacitive sensing characteristics on page
Self-calibration
On power-up, all keys are self-calibrated to provide reliable operation under almost any
conditions. The calibration phase is used to compute a reference value per key which is then
used by the process determining if a key is touched or not. The reference is an average of 8
single acquisitions. As a result, the calibration time of the system can be simply calculated
using the following formula: t
burst period is described in application note AN2547. For a maximum calibration duration
(t
CAL
), please refer to
Section 6.5: Capacitive sensing characteristics on page
CAL
33.
= 8 * Burst_Period. The methodology used to measure the
33.
S
) fitted is assumed to be
inSection 6.5: Capacitive
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33.
QST104
S
.