AT42QT1011 View Datasheet(PDF) - Atmel Corporation
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AT42QT1011 Datasheet PDF : 25 Pages
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3.2 Threshold
The internal signal threshold level is fixed at 10 counts of change with respect to the internal reference level, which in
turn adjusts itself slowly in accordance with the drift compensation mechanism.
The QT1011 employs a hysteresis dropout of two counts of the delta between the reference and threshold levels.
3.3 Max On-duration
The max on-duration of this device is infinite; that is, the device will not automatically recalibrate due to a persistent
detection.
3.4 Detect Integrator
It is desirable to suppress detections generated by electrical noise or from quick brushes with an object. To
accomplish this, the QT1011 incorporates a detect integration (DI) counter that increments with each detection until a
limit is reached, after which the output is activated. If no detection is sensed prior to the final count, the counter is
reset immediately to zero. In the QT1011, the required count is four. In LP mode the device will switch to Fast mode
temporarily in order to resolve the detection more quickly; after a touch is either confirmed or denied the device will
revert back to normal LP mode operation automatically.
The DI can also be viewed as a “consensus filter” that requires four successive detections to create an output.
3.5 Forced Sensor Recalibration
The QT1011 has no recalibration pin; a forced recalibration is accomplished when the device is powered up or after
the recalibration timeout. However, supply drain is low so it is a simple matter to treat the entire IC as a controllable
load; driving the QT1011's Vdd pin directly from another logic gate or a microcontroller port will serve as both power
and “forced recalibration”. The source resistance of most CMOS gates and microcontrollers is low enough to provide
direct power without problem.
3.6 Drift Compensation
Signal drift can occur because of changes in Cx and Cs over time. It is crucial that drift be compensated for,
otherwise false detections, non-detections, and sensitivity shifts will follow.
Drift compensation (Figure 3-5) is performed by making the reference level track the raw signal at a slow rate, but
only while there is no detection in effect. The rate of adjustment must be performed slowly, otherwise legitimate
detections could be ignored. The QT1011 drift compensates using a slew-rate limited change to the reference level;
the threshold and hysteresis values are slaved to this reference.
Once an object is sensed, the drift compensation mechanism ceases since the signal is legitimately high, and
therefore should not cause the reference level to change.
Figure 3-5. Drift Compensation
Signal
Threshold
Hysteresis
Output
Reference
AT42QT1010 [DATASHEET]
9
9542I–AT42–05/2013
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