at42qt113b ATMEL Corporation, at42qt113b Datasheet - Page 9

• Manufacturer Part Number: at42qt113b
• Manufacturer: ATMEL Corporation

3.1.3
3.1.4
3.1.5
3.1.6
9525B–AT42–09/10
Threshold Calculation
Max On-Duration
Detection Integrator
Forced Sensor Recalibration
The internal threshold level is fixed at one of two setting as determined by
settings are fixed with respect to the internal reference level, which in turn will move in
accordance with the drift compensation mechanism.
Table 3-1.
The QT113B employs a hysteresis dropout below the threshold level of 17 percent of the delta
between the reference and threshold levels.
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 timer causes the sensor to perform a full
recalibration (when not set to infinite). This is known as the Max On-Duration feature.
After the Max On-Duration interval, the sensor once again functions normally to the best of its
ability, given electrode conditions. There are two finite timeout durations available via strap
option: 10 and 60 seconds (see
It is desirable to suppress detections generated by electrical noise or from quick brushes with an
object. To accomplish this, the QT113B incorporates a detect integration 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
QT113B, the required count is 3.
The Detection Integrator can also be viewed as a “consensus filter”, that requires three
successive detections to create an output.
The QT113B has no recalibration pin; a forced recalibration is accomplished only when the
device is powered up. However, supply drain is low so it is a simple matter to treat the entire IC
as a controllable load; simply driving the QT113B’s Vdd pin directly from another logic gate or a
microcontroller port (see
source resistance of most CMOS gates and microcontrollers are low enough to provide direct
power without problem. Note that many 8051-based micros have only a weak pull-up drive
capability and will require CMOS buffering. 74HC or 74AC series gates can directly power the
QT113B, as can most other microcontrollers.
Low – 12 counts
High – 6 counts
Gain
Gain Setting Strap Options
Figure
Tie Pin 5 To...
Vss (Gnd)
Table 3-2 on page
Vdd
3-2) will serve as both power and forced recalibration. The
10).
AT42QT113B
Table
3-1. These
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