QST104 STMicroelectronics, QST104 Datasheet - Page 9

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

QST104
3.7
3.8
3.9
3.10
Fast positive recalibration
The device autorecalibrates a key when its signal reflects a decrease in capacitance higher
than a fixed threshold (PosRecalTh) for a defined number of acquisitions (PoseRecalI).
Forced key recalibration
A recalibration of the device may be issued at any time by sending to the QST device the
appropriate I
It is possible to recalibrate independently any individual key using an I
Max On-Duration
The device can time out and automatically recalibrate each key independently after a fixed
duration of continuous touch detection. This prevents the keys from becoming ‘stuck on’ due
to foreign objects or other sudden influences. This is known as the Max On-Duration feature.
After recalibration, the key will continue to operate normally, even if partially or fully
obstructed. Max On-Duration works independently per channel: a timeout on one channel
has no effect on another channel.
Infinite timeout is useful in applications where a prolonged detection can occur and where
the output must reflect the detection no matter how long. In infinite timeout mode, the
designer should take care to ensure that drift in C
remain “stuck on” inadvertently even when the touching object is removed from the sense
field. Timeout durations are not accurate and can vary substantially depending on V
temperature values, and should not be relied upon for critical functions.
Drift compensation
Signal drift can occur because of changes in C
C
are subject to extremes of temperature or humidity, the signal can also drift. It is crucial that
drift be compensated, otherwise false detections, non detections, and sensitivity shifts will
follow.
Drift compensation slowly corrects the reference level of each key while no detection is in
effect. The rate of reference adjustment must be performed slowly or else legitimate
detections can also be ignored. The device compensates drift on each channel
independently using a maximum compensation rate to the reference level.
Once a touch is sensed, the drift compensation mechanism ceases since the signal is
legitimately high, and therefore should not cause the reference level to change.
The signal drift compensation is “asymmetric”: the reference level compensates drift in one
direction faster than it does in the other. Specifically, it compensates faster for increasing
signals than for decreasing signals. Decreasing signals should not be compensated for
quickly, since an approaching finger could be compensated for partially or entirely while
approaching the sense electrode. However, an obstruction over the sense pad, for which the
sensor has already made full allowance, could suddenly be removed leaving the sensor with
an artificially elevated reference level and thus become insensitive to touch. In this latter
case, the sensor will compensate for the object's removal very quickly, usually in only a few
seconds.
S
type and quality, the signal may vary substantially with temperature and veiling. If keys
2
C command or by tying the RESET pin to ground.
X
, C
S
, C
S
, and V
X
, and V
QST touch sensing technology
DD
DD
over time. Depending on the
do not cause the device to
2
C command.
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and
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