3.3 Max On-duration
If an object or material obstructs the sense pad, the signal may rise enough to create a detection,
preventing further operation. To prevent this, the sensor includes a timer which monitors detections. If a
detection exceeds the timer setting, the sensor performs a full recalibration. This is known as the Max
On-duration feature and is set to ~60s (at 3V in LP mode). This will vary slightly with Cs and if SYNC
mode is used. As the internal timebase for Max On-duration is determined by the burst rate, the use of
SYNC can cause dramatic changes in this parameter depending on the SYNC pulse spacing. For
example, at 60Hz SYNC mode the Max On-duration will be ~6s at 3V.
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 QT1010 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 QT1010, 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 QT1010 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 QT1010'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 QT1010 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.
AT42QT1010
© 2017 Microchip Technology Inc.
Datasheet
DS40001946A-page 12
