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
Threshold
Signal
Hysteresis
Reference
Output
The QT1011 drift compensation is asymmetric; the reference level drift-compensates in one direction
faster than it does in the other. Specifically, it compensates faster for decreasing signals than for
AT42QT1011
© 2017 Microchip Technology Inc.
Datasheet
DS40001947A-page 12
