AT42QT1012_10 ATMEL [ATMEL Corporation], AT42QT1012_10 Datasheet - Page 8

• Manufacturer Part Number: AT42QT1012_10
• Description: One-channel Toggle-mode Touch Sensor IC with Power Management Functions
• Manufacturer: ATMEL [ATMEL Corporation]

3.2
3.3
3.4
3.5
3.6
8
Detect Threshold
Detect Integrator
Recalibration Timeout
Forced Sensor Recalibration
Drift Compensation
AT42QT1012
The device detects a touch when the signal has crossed a threshold level. The threshold level is
fixed at 10 counts.
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.
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 ~60s. This will vary slightly with Cs.
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’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 a problem.
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
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 on page
9) is performed by making the reference level track the
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