QST102 STMicroelectronics, QST102 Datasheet - Page 9

• Manufacturer Part Number: QST102
• Description: Capacitive Touch Sensor Device 2 Keys With Individual Key State Outputs
• Manufacturer: STMicroelectronics

QST102
3.9
3.10
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.
Figure 4
change.
Figure 4.
Adjacent key suppression (AKS™)
Adjacent key suppression (AKS™) is a Quantum-patented feature which prevents multiple
keys from responding to a single touch. This can happen with closely spaced keys, or a
scroll wheel that has buttons very near it.
The device supports Locking AKS mode where once a key is considered as “touched”, all
other keys are locked in an untouched state. To unlock these keys, the touched key must
return to an untouched state. Then, the key having the lowest key ID number is declared as
the “touched” one.
S
type and quality, the signal may vary substantially with temperature and veiling. If keys
illustrates an example of the drift compensation algorithm following a temperature
Drift compensation example
Temperature Change
Time
X
, C
Drift Compensation
S
, and V
QST touch sensing technology
DD
over time. Depending on the
Reference Count + PosRelTh
Reference + DeTh
Reference Count
9/29