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QST508CB0 Datasheet(PDF) 11 Page - STMicroelectronics |
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QST508CB0 Datasheet(HTML) 11 Page - STMicroelectronics |
11 / 51 page QST108 QST touch sensing technology 11/51 3.9 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 CS, CX, and VDD do not cause the device to 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 VDD and temperature values, and should not be relied upon for critical functions. 3.10 Drift compensation Signal drift can occur because of changes in CX, CS, and VDD over time. Depending on the CS type and quality, the signal may vary substantially with temperature and veiling. If keys 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. Caution: When only one key is enabled or if keys are very close together, the common drift compensation must be disabled or its rate must be reduced to ensure correct device operation. |
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