Datasheet

3.3 Detect Integrator

It is desirable to suppress detections generated by electrical noise or from quick brushes with an object.

To accomplish this, the QT1012 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 QT1012, the required count is four.

The DI can also be viewed as a “consensus filter” that requires four successive detections to create an

output.

3.4 Recalibration Timeout

If an object or material obstructs the sense electrode the signal may rise enough to create a detection,

preventing further operation. To stop this, the sensor includes a timer which monitors detections. If a

detection exceeds the timer setting, the sensor performs a full recalibration. This does not toggle the

output state but ensures that the QT1012 will detect a new touch correctly. The timer is set to activate this

feature after ~60 s. This will vary slightly with Cs.

3.5 Forced Sensor Recalibration

The QT1012 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 QT1012 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 a 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, nondetections, and sensitivity shifts will follow.

Drift compensation (Figure 3-3) 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 QT1012 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 3-3. Drift Compensation

Threshold

Signal

Hysteresis

Reference

Output

The QT1012 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

AT42QT1012

Datasheet

DS40001948A-page 11